force return to be manifestly typed

[dherman]

Right now the validator is forced to perform inference to compute the result types of the functions. Come up with a policy that makes it possible to compute the function types eagerly.

• returning a variable should be fine without annotation
• we can make floating-point literals obviously float64
• we can make negative integer literals obviously int32
• we can pick a default integer type
• we can make it so that only one return statement has to be obviously typed
• but we may be forced to sometimes require annotated literals

[dherman]

I think for now we should just always require the coercion, even for floating-point literals, to keep the rules simple. Not sure there's much, if anything, we can do about preventing coercions for integer literals.

[dherman]

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