Avoid use of json.NewDecoder

[craigpastro]

Avoid use of json.NewDecoder if not needed.

I was hanging around here and I noticed #303. This is an attempt to address it. Please let me know what you think. Thanks!

Resolves #303.

[oxisto]

Looks good in theory. The extra almost duplicated code is a bit annoying, but the performance gain should be visible. I had some numbers in the original issue, but I will try to do some performance comparisons with this next patch week.

[oxisto]

Numbers look good. As expected, we have a good performance gain (and slightly less allocations).

Before:

BenchmarkParseUnverified/map_claims-8         	 1497346	       792.5 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#01-8      	 1343728	       886.2 ns/op	    2344 B/op	      41 allocs/op
BenchmarkParseUnverified/map_claims#02-8      	 1270246	       939.6 ns/op	    2344 B/op	      41 allocs/op
BenchmarkParseUnverified/map_claims#03-8      	 1211380	      1045 ns/op	    2447 B/op	      46 allocs/op
BenchmarkParseUnverified/map_claims#04-8      	 1528545	       787.7 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#05-8      	 1519468	       796.2 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#06-8      	 1500554	       822.2 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#07-8      	 1485224	       814.9 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#08-8      	 1446320	       830.3 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#09-8      	 1475530	       815.5 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#10-8      	 1493253	       848.2 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#11-8      	 1475796	       818.6 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#12-8      	 1433794	       817.1 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#13-8      	 1292407	       945.9 ns/op	    2344 B/op	      41 allocs/op
BenchmarkParseUnverified/map_claims#14-8      	 1301379	       941.3 ns/op	    2344 B/op	      41 allocs/op
BenchmarkParseUnverified/map_claims#15-8      	 1000000	      1033 ns/op	    2448 B/op	      46 allocs/op
BenchmarkParseUnverified/map_claims#16-8      	 1293612	       925.4 ns/op	    2344 B/op	      41 allocs/op
BenchmarkParseUnverified/registered_claims-8  	 1417246	       844.8 ns/op	    2231 B/op	      35 allocs/op
BenchmarkParseUnverified/registered_claims#01-8         	 1394887	       868.4 ns/op	    2280 B/op	      39 allocs/op
BenchmarkParseUnverified/registered_claims#02-8         	 1286151	       950.9 ns/op	    2344 B/op	      42 allocs/op
BenchmarkParseUnverified/registered_claims#03-8         	 1447845	       822.4 ns/op	    2208 B/op	      34 allocs/op
BenchmarkParseUnverified/registered_claims#04-8         	 1294418	       934.2 ns/op	    2336 B/op	      41 allocs/op
BenchmarkParseUnverified/registered_claims#05-8         	 1378839	       853.6 ns/op	    2224 B/op	      35 allocs/op
BenchmarkParseUnverified/registered_claims#06-8         	 1390922	       860.2 ns/op	    2224 B/op	      35 allocs/op

After:

BenchmarkParseUnverified/map_claims-8         	 1917262	       668.2 ns/op	    1464 B/op	      32 allocs/op
BenchmarkParseUnverified/map_claims#01-8      	 1561243	       714.7 ns/op	    1600 B/op	      38 allocs/op
BenchmarkParseUnverified/map_claims#02-8      	 1691029	       785.0 ns/op	    1592 B/op	      38 allocs/op
BenchmarkParseUnverified/map_claims#03-8      	 1548421	       836.3 ns/op	    1696 B/op	      43 allocs/op
BenchmarkParseUnverified/map_claims#04-8      	 1874676	       614.6 ns/op	    1464 B/op	      32 allocs/op
BenchmarkParseUnverified/map_claims#05-8      	 1958278	       630.7 ns/op	    1464 B/op	      32 allocs/op
BenchmarkParseUnverified/map_claims#06-8      	 1834294	       611.8 ns/op	    1464 B/op	      32 allocs/op
BenchmarkParseUnverified/map_claims#07-8      	 1884421	       638.2 ns/op	    1464 B/op	      32 allocs/op
BenchmarkParseUnverified/map_claims#08-8      	 1887714	       653.2 ns/op	    1464 B/op	      32 allocs/op
BenchmarkParseUnverified/map_claims#09-8      	 1747687	       680.3 ns/op	    1464 B/op	      32 allocs/op
BenchmarkParseUnverified/map_claims#10-8      	 1887813	       658.6 ns/op	    1464 B/op	      32 allocs/op
BenchmarkParseUnverified/map_claims#11-8      	 1830204	       647.6 ns/op	    1464 B/op	      32 allocs/op
BenchmarkParseUnverified/map_claims#12-8      	 1420887	       898.6 ns/op	    2208 B/op	      35 allocs/op
BenchmarkParseUnverified/map_claims#13-8      	 1247702	       953.2 ns/op	    2344 B/op	      41 allocs/op
BenchmarkParseUnverified/map_claims#14-8      	 1000000	      1060 ns/op	    2344 B/op	      41 allocs/op
BenchmarkParseUnverified/map_claims#15-8      	 1000000	      1077 ns/op	    2448 B/op	      46 allocs/op
BenchmarkParseUnverified/map_claims#16-8      	 1227679	      1009 ns/op	    2344 B/op	      41 allocs/op
BenchmarkParseUnverified/registered_claims-8  	 1281345	       897.1 ns/op	    2231 B/op	      35 allocs/op
BenchmarkParseUnverified/registered_claims#01-8         	 1312928	       902.9 ns/op	    2280 B/op	      39 allocs/op
BenchmarkParseUnverified/registered_claims#02-8         	 1236802	      1110 ns/op	    2344 B/op	      42 allocs/op
BenchmarkParseUnverified/registered_claims#03-8         	 1448676	       838.2 ns/op	    2208 B/op	      34 allocs/op
BenchmarkParseUnverified/registered_claims#04-8         	 1000000	      1041 ns/op	    2336 B/op	      41 allocs/op
BenchmarkParseUnverified/registered_claims#05-8         	 1692842	       685.4 ns/op	    1480 B/op	      32 allocs/op
BenchmarkParseUnverified/registered_claims#06-8         	 1792794	       669.0 ns/op	    1480 B/op	      32 allocs/op

For example registered_claims#05-8 and registered_claims#06-8 are the ones not using JSON number, and those seem to be almost 20% faster and less allocations (32 vs 35)