compress/lzw: add Reset method to Reader and Writer

We add a Reset method which clears any internal state of an encoder
or a decoder to let it be reused again as a new Writer or Reader respectively.

We also export the encoder and decoder structs, renaming them
to be Reader and Writer, and we guarantee that the underlying types
from the constructors will always be Reader and Writer respectively.

Benchmark results by reusing the encoder:
on cpu: Intel(R) Core(TM) i5-8265U CPU @ 1.60GHz

name                 time/op
Decoder/1e4-8          93.6µs ± 1%
Decoder/1e-Reuse4-8    87.7µs ± 1%
Decoder/1e5-8           877µs ± 1%
Decoder/1e-Reuse5-8     860µs ± 3%
Decoder/1e6-8          8.79ms ± 1%
Decoder/1e-Reuse6-8    8.82ms ± 4%
Encoder/1e4-8           168µs ± 2%
Encoder/1e-Reuse4-8     160µs ± 1%
Encoder/1e5-8          1.64ms ± 1%
Encoder/1e-Reuse5-8    1.61ms ± 2%
Encoder/1e6-8          16.2ms ± 6%
Encoder/1e-Reuse6-8    15.8ms ± 2%

name                 speed
Decoder/1e4-8         107MB/s ± 1%
Decoder/1e-Reuse4-8   114MB/s ± 1%
Decoder/1e5-8         114MB/s ± 1%
Decoder/1e-Reuse5-8   116MB/s ± 3%
Decoder/1e6-8         114MB/s ± 1%
Decoder/1e-Reuse6-8   113MB/s ± 5%
Encoder/1e4-8        59.7MB/s ± 2%
Encoder/1e-Reuse4-8  62.4MB/s ± 1%
Encoder/1e5-8        61.1MB/s ± 1%
Encoder/1e-Reuse5-8  62.0MB/s ± 2%
Encoder/1e6-8        61.7MB/s ± 5%
Encoder/1e-Reuse6-8  63.4MB/s ± 2%

name                 alloc/op
Decoder/1e4-8          21.8kB ± 0%
Decoder/1e-Reuse4-8     50.0B ± 0%
Decoder/1e5-8          21.8kB ± 0%
Decoder/1e-Reuse5-8     70.4B ± 2%
Decoder/1e6-8          21.9kB ± 0%
Decoder/1e-Reuse6-8      271B ± 3%
Encoder/1e4-8          77.9kB ± 0%
Encoder/1e-Reuse4-8    4.17kB ± 0%
Encoder/1e5-8          77.9kB ± 0%
Encoder/1e-Reuse5-8    4.27kB ± 0%
Encoder/1e6-8          77.9kB ± 0%
Encoder/1e-Reuse6-8    5.22kB ± 0%

name                 allocs/op
Decoder/1e4-8            2.00 ± 0%
Decoder/1e-Reuse4-8      1.00 ± 0%
Decoder/1e5-8            2.00 ± 0%
Decoder/1e-Reuse5-8      1.00 ± 0%
Decoder/1e6-8            2.00 ± 0%
Decoder/1e-Reuse6-8      1.00 ± 0%
Encoder/1e4-8            3.00 ± 0%
Encoder/1e-Reuse4-8      2.00 ± 0%
Encoder/1e5-8            3.00 ± 0%
Encoder/1e-Reuse5-8      2.00 ± 0%
Encoder/1e6-8            3.00 ± 0%
Encoder/1e-Reuse6-8      2.00 ± 0%

Fixes #26535

Change-Id: Icde613fea6234a5bdce95f1e49910f5687e30b22
Reviewed-on: https://go-review.googlesource.com/c/go/+/273667


Trust: Agniva De Sarker <agniva.quicksilver@gmail.com>
Trust: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>