Wasmtime module cache.
The cache uses a per-code "single flight" protocol. The first thread that
misses the LRU for a code hash records that hash in compiling, drops the
lock, and compiles the module. Threads requesting the same hash wait on a
condition variable, while threads requesting other hashes can reserve their
own compile slots and proceed independently.
A CompilePermit represents ownership of one in-progress compile. Dropping
it always removes the hash from compiling and wakes waiters, so both
successful compilation and early errors unblock the next thread.