Feature: Expanded stackfile low-level input and output

Adds the expanded stackfile file format specification and IO library to the engine.  It is currently only accessible via the special internal commands:

    _internal stackdir save <state> to <filename>
    _internal stackdir load <filename> into <variable>

It currently lacks source-info collection (i.e. the `r_source_info` array available via `MCStackdirIOGetState()` is non-useful).