Pad size of TypeId and remove structural equality

[carbotaniuman]

This implements @eddyb's suggestions from #95845

Acouple smaller things could also be done (not sure if I'll be able to help with either):

• manual PartialEq impl for TypeId (i.e. turning off "structural match")

  • this would disallow match-ing on TypeIds (forcing the use of == instead), as to not box ourselves into a corner if address equality is ever needed in the future

• increasing TypeId's size (and cratering that change)

  • should help with breaking misuses (just like comments on this PR describe)

  • it doesn't have to be actual new data, could just be a dead field

    • this is library-only, e.g. TypeId(type_hash::<T>()) -> TypeId(type_hash::<T>(), 0)

Given that the lang-team has already committed to a full length cryptographic hash, increasing the size of TypeId in preparation for a future change to a cryptographic hash should be a simple stepping stone. This also removes structural equality from TypeId for more futureproofing.