Adding methods and changing the API in goish way

[shenwei356]

Hi @brentp ,

This PR includes:

1. Adding methods:
   • Del for deleting a key-value pair.
   • Keys returns a channel for iterating keys.
   • Items returns a channel for iterating all key-value pairs.
   • Size returns the map size.
2. Fixing a bug:
   • rehash() forgets to reset map size, which brought incorrect (bigger) map size and prematurely rehashing.
3. Changing APIs in goish way
   • Get: Function in Java does not supports returning multiple values, so the original implemention uses a NO_VALUE to indicating keys no found. But golang does, so we can just returns another boolean value like builtin map does (value, ok := m[key]).
   • Put does not need returning value.
4. And of cause updated tests and README.

[brentp]

awesome! can you verify that the benchmarks are unaffected by the switch to val, ok ?

[shenwei356]

It seems unaffected.

Previous

BenchmarkIntIntMapFill-16                             10         147056031 ns/op        402522201 B/op        24 allocs/op
BenchmarkStdMapFill-16                                 5         290888868 ns/op        95069708 B/op      73641 allocs/op
BenchmarkIntIntMapGet10PercentHitRate-16           10000            115791 ns/op           40252 B/op          0 allocs/op
BenchmarkStdMapGet10PercentHitRate-16              10000            108940 ns/op            9504 B/op          7 allocs/op
BenchmarkIntIntMapGet100PercentHitRate-16            500           2311054 ns/op          805044 B/op          0 allocs/op
BenchmarkStdMapGet100PercentHitRate-16               100          10592906 ns/op          950572 B/op        735 allocs/op

Now

BenchmarkIntIntMapFill-16                             10         141656399 ns/op        201195776 B/op        22 allocs/op
BenchmarkStdMapFill-16                                 5         287342524 ns/op        95065142 B/op      73611 allocs/op
BenchmarkIntIntMapGet10PercentHitRate-16           20000             59330 ns/op           10059 B/op          0 allocs/op
BenchmarkStdMapGet10PercentHitRate-16              10000            130110 ns/op            9503 B/op          7 allocs/op
BenchmarkIntIntMapGet100PercentHitRate-16            500           2630494 ns/op          402394 B/op          0 allocs/op
BenchmarkStdMapGet100PercentHitRate-16               100          10598248 ns/op          950077 B/op        732 allocs/op

[brentp]

cheers. this is a great improvement to the API.

[shenwei356]

Cheers!