[#670] Make length polymorphic in its return value

Description

Problem: Unsafe.fromIntegral . length has become widespread, but it feels bad each time one has to use it.

Solution: Make length polymorphic. Ensure the cast is safe(-ish) using machinery from FromIntegral (however the cast itself still has to be unsafe because we're casting from Int despite negative values being nonsense). To reduce friction, use the incoherent instance trick to force the return type to Int on ambiguity (otherwise there's a lot of pain with polymorphic integral literals)

Related issue(s)

Resolves #670 (closed)

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