Implement function subtyping

[Chriscbr]

Examples of function subtyping in action:

• any function that accepts a parameter of type T can be assigned to a variable typed as a function that accepts a parameter of type U as long as T is a subtype of U
• any function that returns type T can assigned to variable typed as a function that returns type U as long as U is a subtype of T
• any function that is phase-independent can be assigned to a variable typed as an inflight function as long as the rest of the signatures match
• any function that is phase-independent can be assigned to a variable typed as a preflight function as long as the rest of the signatures match

The subtyping logic is currently located here:

wing/libs/wingc/src/type_check.rs

Line 249 in c3a9534

(Self::Function(l0), Self::Function(r0)) => l0 == r0,

[Chriscbr]

Depends on #1678

[monadabot]

Congrats! 🚀 This was released in Wing 0.5.125.