Support FUNCTIONs and SUBs

[flagxor]

This is needed to support a bunch of the qbasic programs like GORILLA.BAS + NIBBLES.BAS

[FellippeHeitor]

For Gorilla.bas to run as is, DEF FN should also be considered.

[hax]

Could we throw a error message for unimplemented feature now?

(When I see this project I'm excited because it make me recall my childhood, so I searched the source for GORILLA.BAS and try it, but just get the black screen with silence, make me a little frustrating :)

[FellippeHeitor]

@hax: Use QB64 until wwwbasic gets there :-)

[flagxor]

Yeah, would like to get this sorted soon.
It's a largish change as it adds call frames, but getting close.

DEF FN will be part of the same change.

[FellippeHeitor]

The only difference between a FUNCTION and an FN is scope, really, as DEF FN can access local variables. We haven't gotten it working in QB64 so far (we ship a slightly modified version of gorilla.bas that has DEF FN turned into FUNCTION to work).

[flagxor]

Implemented (roughly) in:
95489f9

Will sort out any bugs in other issues.