Change the encoder's hash table values from int32 to uint16.
Change the encoder's hash table values from int32 to uint16.

Doing s/int32/uint16/ in "var table [maxTableSize]int32" saves 32 KiB of stack
space that needed zero'ing. maxTableSize is 1<<14, or 16384.

We couldn't do this before, in commit cc71ae7c, since we didn't have
maxBlockSize = 65536 at the time.

benchmark                     old MB/s     new MB/s     speedup
BenchmarkWordsEncode1e1-4     468.81       465.82       0.99x
BenchmarkWordsEncode1e2-4     32.62        60.23        1.85x
BenchmarkWordsEncode1e3-4     138.36       174.61       1.26x
BenchmarkWordsEncode1e4-4     170.79       172.43       1.01x
BenchmarkWordsEncode1e5-4     131.02       134.25       1.02x
BenchmarkWordsEncode1e6-4     149.33       153.05       1.02x
BenchmarkRandomEncode-4       5933.57      6846.03      1.15x
Benchmark_ZFlat0-4            306.98       310.74       1.01x
Benchmark_ZFlat1-4            194.65       198.74       1.02x
Benchmark_ZFlat2-4            6784.51      8110.98      1.20x
Benchmark_ZFlat3-4            64.06        123.43       1.93x
Benchmark_ZFlat4-4            2102.05      2224.84      1.06x
Benchmark_ZFlat5-4            303.89       307.19       1.01x
Benchmark_ZFlat6-4            132.74       136.37       1.03x
Benchmark_ZFlat7-4            126.50       130.72       1.03x
Benchmark_ZFlat8-4            140.35       143.61       1.02x
Benchmark_ZFlat9-4            121.73       125.62       1.03x
Benchmark_ZFlat10-4           360.89       365.62       1.01x
Benchmark_ZFlat11-4           195.92       199.96       1.02x

FactomProject/snappy-go,FactomProject/snappy-go,golang/snappy,golang/snappy