Add get_function_signature()

Two small points about this commit:
	1) The code only compiles, I haven't tested it
	2) This isn't all the information we would ideally have in the
	   backtrace.  It would be nice to get backtraces like:

	      foo(a int = 24, b text = "this is a long st..")

	   for PL/PgSQL functions.  We probably won't be able to do that
	   for functions written in other languages. :-(