Optimize single block encodes

[klauspost]

With a one type data source:

λ benchstat old.txt new.txt
name                                 old time/op    new time/op    delta
Encoder_EncodeAllSimple4K/fastest-2    31.7µs ± 0%    27.1µs ± 0%  -14.46%        (p=0.000 n=10+10)
Encoder_EncodeAllSimple4K/default-2    49.0µs ± 0%    44.9µs ± 0%   -8.51%        (p=0.000 n=10+10)

name                                 old speed      new speed      delta
Encoder_EncodeAllSimple4K/fastest-2   129MB/s ± 0%   151MB/s ± 0%  +16.90%        (p=0.000 n=10+10)
Encoder_EncodeAllSimple4K/default-2  83.5MB/s ± 0%  91.3MB/s ± 0%   +9.30%        (p=0.000 n=10+10)