z-indexes over heterogeneous BitCollections

[blakehawkins]

(u16, f32).z_index() is not currently possible.

[blakehawkins]

Note that unlike z-indexes over homogeneous types with different masks, z-indexes over heterogeneous types should be right-padded. E.g.:

assert_eq!(
    (FromU8(0b1), FromU16(0b1)).z_index()?,
    0b0000000100000000_0000000000000001  // Not 0b0000000000000000_0000000000000011
)

[blakehawkins]

See nvzqz/bit-collection-rs#1

Need to either refactor BitCollection to enable dyn support, or move to some other bit collection representation.