lib: improve LocationFunc for kernel funcs

[fingolfin]

Before:

gap> LocationFunc(APPEND_LIST_INTR);
""

After:

gap> LocationFunc(APPEND_LIST_INTR);
"src/listfunc.c:APPEND_LIST_INTR"

[frankluebeck]

I wasn't aware of the function LocationFunc. You may want to look at the code of PageSource, try

gap> PageSource(PageSource);

This could already handle many kernel functions, like

gap> PageSource(APPEND_LIST_INTR);

(it is also showing the location in the terminal). PageSource can also show the locations of declarations of operations. Maybe you want to add that to LocationFunc as well.

gap> PageSource(Size);
gap> PageSource(Size, 2);

[fingolfin]

I was aware of PageSource, but it's not useful for me. I use my own function which opens the relevant file in an editor at the right position.

But this all seems irrelevant to this PR: LocationFunc is mainly useful to me to produce error messages, or for other kinds of debug output (e.g. to give a list of locations of methods and where they are defined; indeed, that's what it is used for in ApplicableMethodTypes.

[fingolfin]

I am also aware of the fact that PageSource knows how to locate operations (I was reviewer of the PR adding it, after all ;-), and even thought briefly about adding it to LocationFunc , but since it can return multiple results, it didn't strike me as that useful for LocationFunc, at least without bigger changes to its API. Since this goes way beyond the little itch I want to fix with this PR, I have no interest in pursuing it at this time for LocationFunc.

[ChrisJefferson]

It would be nice to have a little test, just to make sure we don't break this functionality at some point.

[fingolfin]

Just added a test, let's see if it passes CI.