add handle benchmarks

[rogpeppe]

We add a few benchmarks to give us an idea of the costs of this approach
in its various forms. The "Trad" benchmarks show the base cost of the
non-reflective approach.

BenchmarkUnmarshal2Fields                  1000000      1951 ns/op      80 B/op       3 allocs/op
BenchmarkHandle2FieldsTrad                 1000000      1357 ns/op     280 B/op       6 allocs/op
BenchmarkHandle2Fields                      300000      5451 ns/op     504 B/op      11 allocs/op
BenchmarkHandle2FieldsUnmarshalOnly         500000      4047 ns/op     392 B/op      10 allocs/op
BenchmarkUnmarshal4Fields                   500000      3765 ns/op     144 B/op       7 allocs/op
BenchmarkHandle4FieldsTrad                 1000000      1390 ns/op     280 B/op       6 allocs/op
BenchmarkHandle4Fields                      200000      7603 ns/op     616 B/op      15 allocs/op
BenchmarkHandle4FieldsUnmarshalOnly         300000      6060 ns/op     504 B/op      14 allocs/op
BenchmarkHandle2StringFields               1000000      1448 ns/op     176 B/op       4 allocs/op
BenchmarkHandle2StringFieldsUnmarshalOnly  3000000       490 ns/op      64 B/op       2 allocs/op
BenchmarkHandle2StringFieldsTrad          10000000       177 ns/op      32 B/op       1 allocs/op
BenchmarkHandle4StringFields               1000000      1673 ns/op     208 B/op       4 allocs/op
BenchmarkHandle4StringFieldsUnmarshalOnly  2000000       683 ns/op      96 B/op       2 allocs/op
BenchmarkHandle4StringFieldsTrad           5000000       246 ns/op      32 B/op       1 allocs/op
BenchmarkHandle8StringFields               1000000      2188 ns/op     272 B/op       4 allocs/op
BenchmarkHandle8StringFieldsUnmarshalOnly  1000000      1226 ns/op     160 B/op       2 allocs/op
BenchmarkHandle8StringFieldsTrad           3000000       472 ns/op      32 B/op       1 allocs/op
BenchmarkHandle16StringFields               500000      2882 ns/op     400 B/op       4 allocs/op
BenchmarkHandle16StringFieldsUnmarshalOnly 1000000      1950 ns/op     288 B/op       2 allocs/op
BenchmarkHandle16StringFieldsTrad          2000000       686 ns/op      32 B/op       1 allocs/op

[urosj]

Could we add benchmark for some real-example json that we use in CS?

[urosj]

Otherwise, 👍
Thank you for this!

[mhilton]

👍 LGTM

[rogpeppe]

Will add JSON benchmark in a subsequent PR.