MINOR: sym: add an in-process symbol table for backtraces

MINOR: sym: add an in-process symbol table for backtraces

Build an address-sorted table at boot and publish immutable snapshots for
lock-free, signal-safe lookups. Seed it with the existing known functions.
The table is built while the process is still starting and no thread is
 running yet, so the build needs no locking.
The table is sorted using a radix sort.