Limit number of arguments for a function to `u16::MAX`

[flip1995]

Kind of funny PR:

• Fix ICE for functions with more than 65535 arguments rust-lang/rust#88733

This limits the number of arguments that can be used by a function to 65535 = u16::MAX and emits an error if more arguments are used.

I think this is currently hard to implement a test for, since it practically requires macro expansion, but this is something that should be tested in the future.

[bjorn3]

I think this can be considered an implementation detail just like the 4GB limit on the total size of all source files loaded in by a single rustc invocation from both the current crate and all dependencies. (spans use 32bit ints to indicate the start and end within the source map, which is a concatenated string of all sources) I won't oppose adding this limitation to gccrs too though.