Rewrite trap loading to use generic_factory

[Coolthulhu]

Should fix http://smf.cataclysmdda.com/index.php?topic=14294.0
I can't tell for sure because the bug is most likely related to vector location and thus compiler dependent, mod dependent and even runtime memory dependent.

The trap loading code was unusual and possibly fragile. It might lead to a situation where string and int ids do not match, which would explain the issue in the linked forum thread.

Using generic_factory ensures that even if it is wrong, it is consistently wrong and only needs to be fixed in one place.
Also minor cleanup:

• Using std::function instead of function pointers
• trapfunc becomes a namespace (was struct)
• Changed the trap_function_from_string from an if stack to an unordered map (saves lines, improves readability)