Improve monomorphization cache design for full optimality

[tritao]

Attaching some context from @emilyaherbert how we we can do this:

In general, most generic functions are called without type arguments, so most of the time this check will fail, missing valid cache opportunities.

Right now when type checking a function application, the compiler:

• retrieves the function
• monomorphizes it
• creates an error if an incorrect number of arguments are provided and early exists
• type checks the arguments to the function, find the types of the arguments, and simultaneously unifies the types of the arguments with the expected types of the function parameters

Considering that most generic functions are called (and monomorphized) without type arguments, a good high-level design for using the cache might look like:

• type check the arguments to the function, collect the types of the arguments
• retrieve the function
• potentially monomorphize, depending on the type of the arguments and the type arguments themselves
• create an error if an incorrect number of arguments are provided and early exist
• unify the types of the arguments with the types of the function parameters

This would allow us to use the types of the arguments in monormorphization

[emilyaherbert]

Closing in favor of grouping this issue with #2636.