Improvements to type inference + bug fixes

[thomasfanell]

Notable changes:

• Improvements in type inference
• null-coalescing operator is no longer evaluating the LHS twice

In some cases, we are still required to qualify S, such as when S is used in an argument:

// this works
map: func <S> (function: Func(T) -> S) -> S
// error Not enough info to resolve return type S of function call
fold: func <S> (function: Func(T, S) -> S, initial: S) -> S
// this works
fold: func <S> (S: Class, function: Func(T, S) -> S, initial: S) -> S