Rewrite cut_bytes(), more performant than GNU

Creates BufReader in buffer.rs. BufReader uses a stack allocated buffer
to read into and returns selected slices into the buffer. This does away
with any dynamic allocations in the 'newline loop.

1.5 to 2.5 more performant than previous version.
1.5 to 2.0 times more performant than GNU.
Merge pull request #345 from polyphemus/cut-bytes-rewrite

Rewrite bytes cutting for cut