Allow a proc with one in-out (!) parameter to be called as a function producing the new argument value

[pschachte]

This is the dual of the feature that allows a function to be called as a statement with one argument provided in in-out (!) mode, eg, !x + 1 to increment x. This is to support the principle I call irrelevance insensitivity, which says that unimportant decisions should not have important consequences. So deciding that a proc modifies an argument rather than producing a new value, or vice-versa, should not be important. This request asks that a decision to modify an argument should easily be changed to instead produce a new value.

So if p is declared to have mode p(!x:int, y:int), we'd like to be able to use p(a, b) as an expression whose value is what would be the final value of a if we called p(!a, b), but a is left unchanged. Ie, it should be equivalent to t where {?t=a ; p(!t, b)}. This would only work in cases where there is a single in-out argument, and all the others are inputs.