Fix reduce/reduce conflict

[liam923]

#124 introduced a reduce/reduce conflict to the parser. Although menhir raised a warning, it did not report it as an error. So I didn't notice the issue until importing the changes to our internal codebase. The conflict was simple: a new rule for parsing _ in an overwrite statement conflicted with Merlin's existing parser support for _s as typed holes.

[liam923]

I looked at Menhir's docs, and there isn't an easy way to raise the level of the warning to an error. There is a --strict flag that seems to report all warnings as an error, but there are other existing warnings:

File "src/ocaml/preprocess/parser_raw.mly", line 1162, characters 29-36:
Warning: the token COMMENT is unused.
File "src/ocaml/preprocess/parser_raw.mly", line 1163, characters 30-39:
Warning: the token DOCSTRING is unused.
File "src/ocaml/preprocess/parser_raw.mly", line 1165, characters 7-10:
Warning: the token EOL is unused.
File "src/ocaml/preprocess/parser_raw.mly", line 1060, characters 7-22:
Warning: the token GREATERRBRACKET is unused.
File "src/ocaml/preprocess/parser_raw.mly", line 1235, characters 0-9:
Warning: the precedence level assigned to GREATERDOT is never useful.

So an option would be to use --strict and either fix or ignore those warnings.