[cbmarini]

This PR fixes #828 by correcting how PushHide/PopHide interact with Hide/UnHide.

The intended rules are:

• If PushHide hides F1 and a later Hide hides F2, UnHide; activates only F2; F1 stays hidden.
• PopHide; activates both F1 and F2.
• Expressions hidden before that PushHide stay hidden.

An example:

    Local F1 = 1;
    .sort
    PushHide;
    Local F2 = 2;
    .sort
    Hide;
    Local F3 = 3;
    .sort
    PushHide;
    Local F4 = 4;
    .sort
    Hide;
    .sort
    PopHide;
    #message Active expressions: `activeexprnames_'
~~~Active expressions: F3,F4
    .sort
    UnHide;
    #message Active expressions: `activeexprnames_'
~~~Active expressions: F2,F3,F4
    .sort
    PopHide;
    #message Active expressions: `activeexprnames_'
~~~Active expressions: F1,F2,F3,F4
    .end

A separate comment on the issue reports that Hide F4; was silently accepted even though F4 was not a declared expression. This PR also resolves that issue.

Question: Do we need to extend PushHide with PushHide [function-name] as well?