Fix breakpoint_manager not using its own retq finding function

Looks like there's a lot of duplicated code for the refactor
to clean up.

Finally, instead of having to hardcode a constant, the length of
the code to disassemble in search of a RET instruction is discovered
at runtime.

Also update PDFMainBreakpoint after moving set_hammer_retq_breakpoints() to BreakpointManager

Remove finished TODO entry

- final names TBD
- TODO: GDB commands for highest per-arena and total mem use

- Commented out messages for memory allocations outside a parser context
- Commented out fallback for when a parser vtable can't be found in the dictionary, as that should be an error
- Added debug line printing the address that was not found in the vtable name->address dictionary

Extracting parser environment information will be done through the GDB Value API

- handle parser names being None in HammerParserBacktrace
- figured out subscripting, so the stringly typed code in the Env
classes can be replaced

Also removed an unused dict