The main goal is to make the parser more reusable to be used for when-clauses, instead of the expression parser. A side goal has been to make it more readable by moving the construction of some untyped expression as method on UntypedExpr. Doing so, I got rid of the extra temporary 'ParseArg' type and re-used the generic 'CallArg' instead by simply using an Option<UntypedExpr> as value to get the same semantic as 'ParseArg' (which would distinguish between plain call args and holes). Now the chained parser is in a bit more reusable state.
Alleviate a bit more the top-level expression parser. Note that we
probably need a bit more disciplined in what we export and at what level
because there doesn't seem to be much logic as for whether a parser is
private, exported to the crate only or to the wide open. I'd be in favor
of exporting everything by default.
KtorZ