formality-core parse should permit customizable "commit" point

[nikomatsakis]

If you have a grammar like this...

Expr = Expr . `await`
         | Place 

Place = Id $*Projections

Projections = `.` FieldId

...then a.await will be parsed as a parse error by Place. The reason is that it parses a as an Id (correctly) but when it sees the . it commits to that being followed by a projection. Right now the rule is that we commit if we consumed anything but whitespace. We should let users define the "commit" point.