This works by walking a backtrace until the currently executing
native executable is found, and then mapping the native address
to its bytecode instruction.
Previously every file that included Executable.h (which is pretty much
most LibJS and LibHTML files, given that VM.h needs it) had the whole
definition of LibRegex, which was slowing down source parsing.
This is a specialized string table for storing identifiers only.
Identifiers are always FlyStrings, which makes many common operations
faster by allowing O(1) comparison.