Fail earlier and better on doomed parses

[masak]

I just fixed two specific cases, namely when my and sub are followed by a non-identifier. Instead of backtracking and trying other rules (usually statement:expr, which is very general), the parser should just give up directly with a <.panic> call.

See these two commits for inspiration on how to do this.

This issue can be closed when the parser has been combed for other opportunities like this, and <.panic> calls installed. As much as possible, each such <.panic> should be accompanied by a test in t/syntax/errors.t.

[masak]

I'd say #94 is really only a sub-case of this one.

[masak]

This issue now basically means "have decent compiler error messages", which I still want, but...

...one aspect of that is to think about "secondary source locations". That is, when a call turns out to be impossible, also link to (and show file/line information for) the function(s) whose parameters that call's arguments don't match up with.