don't iterate twice over uncounted seqs, and use explicit key/val accessors for kv-collections, appears to give ~25% improvement in 'freeze'
add fast-path encoding for long, doubles, keywords, and strings
Update benchmarks
Merge branch 'ztellman'
Revert "add fast-path encoding for long, doubles, keywords, and strings"

This reverts commit a92c493375d95996b999eed0736e28016bd94814.

As per https://github.com/ptaoussanis/nippy/pull/31, tests appear to be
inconclusive about the effetcs of this commit. Candidate for future
reconsideration.
v2.3.0
Add README link to mpenet's LZ4 compressor