[pointer] Add separate PtrInner (#1891)

`PtrInner` carries all invariants which are not controlled by type
parameters. Since `PtrInner` does not promise to uphold aliasing,
alignment, or validity, we can move some utility methods to `PtrInner`
which previously were responsible for maintaining invariants orthogonal
to their purpose.

Makes progress on #1892 (still needs to be fixed on v0.8.x)
Closes #1890

gherrit-pr-id: I1c2d4a54bc1d73d3079d043c2bc393a8967f44f6

Author: joshlf