Add ability to transpose Unalign<Cell<T>> and Cell<Unalign<T>>

[joshlf]

Add methods with signatures roughly like the following (names to be bikeshedded):

impl<T> Unalign<Cell<T>> {
    pub const fn transpose_cell(self) -> Cell<Unalign<T>> { ... }
    pub const fn as_transposed_cell(&self) -> &Cell<Unalign<T>> { ... }
    pub const fn as_transposed_cell_mut(&mut self) -> &mut Cell<Unalign<T>> { .. }
}

Note that this might also be handled if we implement generic transposition in #196.

[joshlf]

Not clear that there's a use case for this.