lint: add internal mutability lint.

This picks up any use of types with internal mutability (containing
`Unsafe`) in function/method calls, since these may cause things to be
modified unexpectedly.

This lint is like the "heap memory" lints, in that using internally
mutable types is perfectly valid in many instances, but it is easy to
expose the compiler's internal knowledge for the cases when someone does
wish to keep track of everything being mutated.