Fix "not match" and "not if" causing a syntax error

[adri326]

This pull request fixes issue #2988, where not if, not for and not while resulted in a parse error, even though these should technically be valid.

Tests have been included to prevent this issue from happening again in the future.

[adri326]

For the record: adding cond to the atom rule caused a collision between match's guard expressions and the regular condition expression (if this then that end), as both caseexpr and prefix rules end up using the atom rule.

The only viable fix I could come up with is to separate the rule tree that caseexpr and prefix use, such that caseexpr's tree does not contain the cond rule.

5 new rules had to be created, each being a slightly modified copy of their respective, already existing rule in prefix's rule tree:

• casepattern
• caseparampattern
• caseprefix
• casepostfix
• caseatom (which does not contain cond)

[SeanTAllen]

@jemc can you give this a review?