Create function engine

[emilyaherbert]

In order to stop inlining function bodies and eliminate duplicated monomorphization during type checking, we will need to have the ability to refer to function bodies abstractly. Using a function engine during type checking would allow us to do this.

For example, given an untyped function declaration add(a: u64, b: u64) -> u64, we would create a typed function declaration and add it to the function engine. During calls to add instead of inlining the function body, we add a reference Body(add, vec!(u64, u64)), and that will serve as a pointer to the body of add. The return type for the call to add would be ReturnType(add, vec!(u64, u64)).

Given an untyped function declaration sub<T>(a: T, b: T) -> T, we would create a typed function declaration and add it to the function engine. During calls to sub(0u8, 1u8) instead of inlining the function body, we add a reference Body(sub, vec!(u8, u8)). The return type for the call to sub would be ReturnType(sub, vec!(u8, u8)). We do not perform monomorphization at this time.

After type checking has completed, before IR. An additional pass (we will expand upon the pass added here #1820) will be used to resolve all types, monomorphize functions, and resolve all function bodies. During this pass, references like Body(sub, vec!(u8, u8)) will be used to locate the definition of sub that takes types u8, u8 (or monomorphize sub and create one, if it does not already exist).

The implementation of the function engine will eventually be expanded to a declaration engine (#1692), which will be used to implement #1819.

Blocked by:

• Add a pass between type-checking and IR to resolve types #1820

Closes:

• Monomorphization implementation is not true monomorphization #862
• Refactor type-checking to not inline everything #1557

Unblocks:

• Implement trait constraints #970
• Handle where clause for data structures. #1159
• Handle where clause for impl traits #1162
• Handle where clause for functions. #1163

[emilyaherbert]

Closing in favor of #1692