FMSLogo Code

Fix a recent regression in FIRST where it could leak a value provided by :erract.

Fix a recent regression in BUTFIRST where it could return a freed NODE*.
Calling gcref(arg) could inadvertantly free val if it were a list.
This change also adds some more comments and an assert around the memory
management contact of validate_argument.

Change BUTFIRSTS so that, on bad input, it considers its entire input bad, not just one element.
This is consistent with how other procedures work.
The logic was adpated from lfirst() and lfirsts().

Change FIRSTS so that, on bad input, the entire input is replaced instead of the bad element.
This is consistent with how other procedures work.

I had thought the code would use validate_argument before entering the
parsing loop, but I didn't like the idea of making two passes over
the list. Instead, the code optimistically collects the output and,
on error, cleans up and gets a whole new input.

Overall, the code didn't turn out clean as I had hoped. Still, I
prefer the new behavior, so I'm going to keep it, and probably apply
similar code to BUTFIRSTS.

Note that Bug #611 has been fixed.

Note that Bug #93 was fixed in FMSLogo 8.0.0.

Fix a typo in a comment

Refactoring: Extract validate_argument and use it in FIRST, BUTFIRST, LAST, BUTLAST.
This is an alternate form of validating an argument that doesn't modify the args list.

For 25 years, I had assumed that modifying the list was useful, like maybe it sped
things up the next time, but the argument is list recreated each time a primitive
is called, so changing it does nothing. I speculate that the reason the argument
list is modified is a cheap form of memory management for any new/converted arguments.

The code base will be easier to undersatnd if doesn't modify the argument list for
non-obvious reasons. This code base will also be easier to understand if it relies
on the return value from functions instead of calling functions that sometimes modify
their arguments.

Create separate procedures for a smoke test and bad input tests for BUTLAST.
This is consistent with how tests for similar procedures are structured.

Add separate functions for a smoke test and a bad input test of LAST.
This follows the structure of the tests for similar functions (FIRST, BUTFIRST, etc)