cmp: Make memtable key comparisons ~4x faster. Similar to the internal key comparison improvement: Only parse expensive FixedInts when required. This change brings rusty-leveldb within 10% of the original for write-heavy workloads and 25% for iterating workloads.

syntax: glob

*.bk
*.orig
*.swp
Cargo.lock
kcov-out
target

# LevelDB test instances
LOG{,.old}
LOCK
CURRENT
*.ldb
*.log
MANIFEST-*