fix: Parse error when --{ looks like subtraction

[dezgeg]

Minimal failing example:

n=n+n
--{
f()
--}

This fails to parse because '--{' in this context looks like subtraction by a negated table constant.

Also, the parser currently allows 'f(--{})' (f called with double-negated table constant), forbid that as well.

[vsariola]

Thanks! I'll look into this and merge asap

[vsariola]

LGTM, thanks! Your tests fail without the fix, and with the fix, everything passes & corpus parses ok, so seems legit.