Create leaner encoder interface for fast levels.

[klauspost]

Separate fast and "standard" compression into a shared interface.

This mainly makes a difference when encoder is used without "Reset".
With PR applied:

file    reset   out level   files   insize  outsize millis  mb/s
sites   false   gzkp    -2  21920   192640560   127352920   2703    67.95
sites   false   gzkp    1   21920   192640560   66222720    3771    48.71
sites   false   gzkp    2   21920   192640560   65271280    4326    42.46
sites   false   gzkp    3   21920   192640560   64362680    4839    37.96
sites   false   gzkp    4   21920   192640560   63061440    4808    38.20
sites   false   gzkp    5   21920   192640560   59535120    8405    21.86
sites   false   gzkp    6   21920   192640560   58412040    8691    21.14
sites   false   gzkp    7   21920   192640560   57069720    9542    19.25
sites   false   gzkp    8   21920   192640560   56384720    10511   17.48
sites   false   gzkp    9   21920   192640560   56011840    13240   13.88

Without:

file    reset   out level   files   insize  outsize millis  mb/s
sites   false   gzkp    -2  21920   192640560   127352920   6235    29.46
sites   false   gzkp    1   21920   192640560   66222720    7167    25.63
sites   false   gzkp    2   21920   192640560   65271280    8067    22.77
sites   false   gzkp    3   21920   192640560   64362680    8670    21.19
sites   false   gzkp    4   21920   192640560   63061440    8659    21.22
sites   false   gzkp    5   21920   192640560   59535120    8441    21.76
sites   false   gzkp    6   21920   192640560   58412040    8705    21.10
sites   false   gzkp    7   21920   192640560   57069720    9652    19.03
sites   false   gzkp    8   21920   192640560   56384720    10480   17.53
sites   false   gzkp    9   21920   192640560   56011840    12964   14.17

[klauspost]

=== RUN   TestFile10
panic: runtime error: slice bounds out of range

goroutine 186 [running]:
panic(0x545820, 0xc042004060)
        c:/go/src/runtime/panic.go:500 +0x1af
github.com/klauspost/compress/flate.(*compressor).fillWindow(0xc043c48000, 0xc044df88d6, 0x4000, 0x572a)
        d:/dev/gopath/src/github.com/klauspost/compress/flate/deflate.go:236 +0x2d0
github.com/klauspost/compress/flate.(*Writer).ResetDict(0xc0438ea030, 0x6074c0, 0xc042070070, 0xc044df88d6, 0x4000, 0x572a)
        d:/dev/gopath/src/github.com/klauspost/compress/flate/deflate.go:860 +0xd0
github.com/klauspost/pgzip.compressBlock(0xc0438fc000, 0x0, 0x4c4b4, 0xc044df88d6, 0x4000, 0x572a, 0x0, 0x0, 0x0, 0x0, ...)
        d:/dev/gopath/src/github.com/klauspost/pgzip/gzip.go:402 +0x1bb
created by github.com/klauspost/pgzip.(*Writer).compressCurrent
        d:/dev/gopath/src/github.com/klauspost/pgzip/gzip.go:243 +0x377
exit status 2
FAIL    github.com/klauspost/pgzip      0.084s

[klauspost]

Replaced by #107