Instead of using bytes.Map which is returning a copy of the provided []byte,
use a custom in-place implementation, as the bytes.Map call is taking around
25% of rss.Parse
io.ReadAll is growing the underlying buffer progressively, while
io.Copy is able to allocate it in one go, which is significantly faster.
io.ReadAll is currently accounting for around 10% of the CPU time of rss.Parse
This will allow to make use of func (*Reader) Seek, instead of re-recreating a
new reader. It's a large commit for a small change, but anything to simply the
reader/buffer/ReadAll/… mess is a step in the right direction I think, and it
should enable more follow-up simplifications.
