gc2stack: Remove unused safeToDelete mechanism.
gc2stack: Replace return type bool with proper enum flag.

This makes the code much easier to read.
gc2stack: Extract static size checking code into helper function.
gc2stack: Move TypeInfo handling code into FunctionInfo subclass.

This is to properly support calls that don't involve TypeInfo
at all, like it is already the case for _d_newclass, and will be
for _d_allocmemory.
gc2stack: Handle _d_allocmemory.

This allows us to clean up after inlining closures.
Merge pull request #532 from klickverbot/gc2stack-fixes

gc2stack: Cleanup, handle _d_allocmemory.