Apply heuristics for identifying functions when no symbols are present

[lordmilko]

IDA Pro is able to identify functions even when no symbols are present. Even when symbols are present, sometimes things aren't actually functions after all (e.g. the many bad functions in System.Data)

Ghidra utilizes a series of heuristics for identifying the locations of functions. Consider implementing this functionality as well. Note that we've seen cases where a function began with sub against rsp or something, so we need to make sure that not only that such functions are identified as well, but that all functions listed in symbols are identified as functions using this heuristic based approach