Check for missing return statement

[jacqueswww]

- What I did

Partial fix for #590.
Check for a minimum of 1 return statement if specified.

- How I did it

Added a count in the function context.

- How to verify it

@public
def foo() -> num:
        pass

- Description for the changelog

- Cute Animal Picture

[fubuloubu]

LGTM for now, obviously will have to rewrite it when we do the branching check instead.

Actually, can you add a TODO note to that effect in the parser code?

[DavidKnott]

Looks like a good first step, particularly because making sure every endpoint has the correct return is significantly more logic. I'd love to see a test or two with conditional returns to make the limits of this fix clearer.

[DavidKnott]

LGTM, merging it in.

[fubuloubu]

Since we have the correct algorithm in #590 according to @yzhang90, perhaps we can upgrade the check now?