Fix and optimize HTTP method parsing

The old parser used a manually unrolled state machine
and was broken due to upstream rust issues with match
statements.

The new parser is a read into a stack-allocated buffer
followed by a single match on the contents of that
buffer.

This significantly improves the benchmarks for method
reads by almost 30%, in addition to working around
the upstream rust issues with reordering match blocks.
Merge pull request #84 'optimize-method-parsing'