987: Refactor maybe_item to use Marker argument r=pcpthm a=pcpthm
As suggested at <https://github.com/rust-analyzer/rust-analyzer/pull/980#issuecomment-473659745>.
For expression paring functions, changing signature
- from `fn(&mut Parser) -> Option<CompletedMarker>` to `fn(&mut Parser, Marker) -> Result<CompletedMarker, Marker>`
- from `fn(&mut Parser) -> CompletedMarker` to `fn(&mut Parser, Marker) -> CompletedMarker`
is my plan.
Co-authored-by: pcpthm <pcpthm@gmail.com>
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