Add `debug.caller()`

[s-hadinger]

Add debug.caller()with the following semantics:

• debug.caller(nil) -> function or nil return current function
• debug.caller(int) -> function or nil return the nth caller in chain (default is 1).

One typical use-case is to get the name of the current function, or the name of the caller function. Note: use introspect.name() to get the name of a function.

Example:

def guess_my_name()
    import debug
    import introspect
    var func = debug.caller()
    var func_name = introspect.name(func)
    print(func_name)
end
guess_my_name()
# prints: guess_my_name