Process uncurried application in the type checker.

[cristianoc]

• Do the processing of uncurried application in the type checker.
• Add support for default arguments in uncurried functions.
• Add custom error messages for uncurried application.
• Uncurried pipe processing does not require special handling.

The current encoding is based on arity. To support default arguments one cannot just look at the number of supplied arguments, but to know whether the required arguments are supplied one needs to inspect the function type.

[cristianoc]

This is an example that currently crashes the compiler (all versions):

let foo = (. ~x) => { let _ = (); (~y) => x+y }

let d = foo(. ~y=3)

Illustrates that it's not enough to count the number of arguments supplied. Here the number of arguments, 1, is correct, but the application looks further in the type than it should.

Handling the application in the type checker gives enough flexibility to handle this issue with a small change later on.

[cristianoc]

Handling the application in the type checker gives enough flexibility to handle this issue with a small change later on.

Here: 173989a

[cristianoc]

Test that this does not break existing uncurried tests in .ml files: #5837