New long EvalFreeNumExpr (expression_t *pfixlist) which automatically releases expression when evaluated

Most code uses

constant = EvalNumExpr (pfixexpr)

then followed by

FreeNumExpr (pfixexpr)

Implement a new

EvalFreeNumExpr (expression_t *pfixlist)

which wraps this functionality as one atomic operation, resulting in cleaner code.

Preserve original EvalNumExpr() since this is used commonly in pass2 and module linking for post-processing of expressions.