Prevent hidden variables from interfering with closures

[dstelzer]

Every div, span, and certain other constructions creates a hidden variable that's used for internal purposes.

These hidden variables should not be considered when determining if the inputs to a closure are bound or not.

For example, this source:

(style class @red) color: red;
(style class @green) color: green;

(interface (this should be bound $<This))
(this should be bound $This) $This

(program entry point)
	(query { (span @red) Red! })
	(query { (span @green) Green! })
	($B = 123)
	(query { (this should be bound $B) })

With the current compiler, it produces this:

Warning: closure-span.dg, line 11: Parameter #1 of (this should be bound $) can be (partially) unbound, which violates the interface declaration at closure-span.dg:4.
Warning: closure-span.dg, line 11: Parameter #1 of (this should be bound $) can be (partially) unbound, which violates the interface declaration at closure-span.dg:4.

Because the red and green spans introduce hidden variables that mean ( invoke-closure $ $ $) does not have fully-bound parameters.

With this PR, those hidden variables are not included in the signature of ( invoke-closure $ $ $) and thus will not be considered by the optimizer. Thus, no warnings.

This was previously reported as #291 and reported as fixed in #292 . However, that PR only fixed it for the debugger. This PR fixes it for the compiler as well.

[dstelzer]

Fixes #291 properly this time