Check declared variable's types with equality instead of unification

[jfecher]

If a variable or function is declared with a type signature it is currently unified against the type of the right hand side expression instead of simply checked for equality. This means that the following snippet:

foo (x: a) : a = x + 1

Type checks despite the actual inferred type of foo being Int a -> Int a. To fix this, we'll need to check for type equality if a type signature is already present, and fallback to unification only if it is not. When checking type equality we'll need to be careful to call follow_all_bindings on each type to ensure any type variable links are already followed, otherwise they can fail the equality check on otherwise equal types.

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I'm marking this "good first issue" mostly for those already familiar with Hindley Milner type systems as it only requires learning part of Ante's type checking pass.

[jfecher]

Fixing this should also fix #168