fix: function and arg size book-keeping for TestSub recursion

[jodavies]

When TestSub is called recursively, correctly update the nested sizes. Rely on the recursion to fix the sizes at each level, except when an argument term disappears completely; then we fix up the whole nesting stack and continue processing further argument terms.

This fix was prepared with the help of an LLM report: https://gist.github.com/tueda/9d71a87e04dcc1f51fbbb432ae1af903

Its findings are in line with previous debugging efforts by me, but I did not figure out a proper fix at the time.

In addition:

• completely remove the "subsubveto" mechanism, which appears to be a previous workaround, required due to the incorrect size updates
• add a check that we do not exceed the FunctionLevels setup parameter, and give an error message for the user if so.
• check RecFlag has the same value on entry and when returning from TestSub

---

This fixes #367 and #741

[jodavies]

Tentatively, this is also a 1% performance improvement in forcer, forcer-exp and maybe hyperform.

[coveralls]

coverage: 63.19% (+0.004%) from 63.186% — jodavies:issue-741-gpt into form-dev:master

[jodavies]

I put back subsubveto, since @tueda is not sure about it. I think it would help if we could find a FORM script which would hit the following code, in the case that AN.subsubveto == 1:

form/sources/proces.c

Lines 741 to 748 in b143fc2

	if ( t[3] == 0 ) {
	t1 = r + r[1];
	t2 = term + *term;
	*term -= r[1]+t[1];
	r = t;
	while ( t1 < t2 ) *r++ = *t1++;
	goto ReStart;
	}