Type of lambda without context should not be inferred as `Any`

[gvanrossum]

See #2567 (comment):

A lambda should always be at least least inferred as Callable (with a specific number of arguments). We know as much about it as we do about an unannotated def. This is really strange:

def f(x): return x
g = lambda x: x
reveal_type(f)
reveal_type(g)

Gives

__tmp__.py:3: error: Revealed type is 'def (x: Any) -> Any'
__tmp__.py:4: error: Revealed type is 'Any'

I think they should reveal the same type.