Encoder speedups

This inlines the readUint32 and replaces a call to copy() with a for
loop.

Results from benchmarks ported from snappy-go:

benchmark                  old ns/op    new ns/op    delta
BenchmarkLZ4Encode          14503246     12714238  -12.34%
BenchmarkWordsEncode1e3        70797        67669   -4.42%
BenchmarkWordsEncode1e4       250701       211082  -15.80%
BenchmarkWordsEncode1e5      1862315      1498543  -19.53%
BenchmarkWordsEncode1e6     15575586     12498643  -19.75%

benchmark                   old MB/s     new MB/s  speedup
BenchmarkWordsEncode1e3        14.12        14.78    1.05x
BenchmarkWordsEncode1e4        39.89        47.37    1.19x
BenchmarkWordsEncode1e5        53.70        66.73    1.24x
BenchmarkWordsEncode1e6        64.20        80.01    1.25x
Merge pull request #10 from dgryski/encode-speedups

speedup the encoder ~15%