Check that function return types inferred in documentation make sense

[stelmo]

This makes it easier to work with them in different packages

[exaexa]

The problem with function type annotations is that it results in extra convert() call in the end, which might and might not be what we actually want. (read: it's not an annotation at all)

[stelmo]

hmm that sucks :/ It is really helpful to know what I can expect when working from another package