Allow merge x y z

[Nadrieril]

Currently, parsing merge x y z is a parse error, and a surprising one
since the syntax resembles function application. This allows it.

Additionally, this clarifies that Some takes one argument and not
many, though without changing the grammar semantics surrounding it.

[Gabriella439]

I would suggest fixing this in a different way, by adding merge and Some as additional cases to the application-expression rule since from a syntactic level they are conceptually behaving the same as function application. In other words, something like this, similar to how constructors worked before we removed it in #385:

application-expression =
      [ (merge / Some) whsp1 ] import-expression *(whsp1 import-expression)

There are two reason I suggest this:

• I think its conceptually cleaner that any "function-application-like" (such as constructors/merge) goes under the application-expression rule
• This formulation also prevents something like Some x Some y from being grammatically valid

[Nadrieril]

Ah you're right, my PR allows f Some x but is parsed as f (Some x) which is unexpected

[Nadrieril]

Fixed, but I still wanted to make explicit the number of arguments needed for Some/merge