Preliminary work towards parse-tree conversion of expression parse-trees.
PEG rules for Constructors applied to tuples, for raise and for if-then-else
Attempts at parse-tree conversion for constructors applied to tuples.
Fix the PEG-parsing of tuples.
Handle parsing and conversions to ASTs for lots of expression forms.

Omissions are case-expressions, let-expressions, function literals and
raise expressions (I PEG-parse the latter but don’t convert it yet).
Correct grammar for patterns.
Merge remote-tracking branch 'origin/master' into modules