Fix a bunch of bugs introduced by the latest round of LF improvements, mainly block refcounting edge cases that went missing and some silly pointer type confusions.
Some low-hanging fruit from performance profiling: don't check is-malloc-fn or is-blacklisted the expensive way every time, and share an allocator for Multis since the default allocates 4K slabs (!)
Save a bunch of pointless string formatting by producing verbose failure reports only when requested
Remove some pointless PB copies from the build-from-bytes path
Add new function that does a one-shot get-pb-and-merge-with-other since this is a common pointless copy.
Scratch another hot-path copy
Remove more IVS copies
Quieten
Switch from DenseMaps to vectors for storing local frame maps.
Reserve space to reduce reallocs
Inline small function (10% perf gain)