Revisit "unplanned for now" section of ffi_coverage ignorelist post-1.0

[Manishearth]

The FFI coverage test has the following section of ignores:

https://github.com/unicode-org/icu4x/blob/90db7c6a5ef3d64e969850c2655e8b9e45948c81/ffi/diplomat/ffi_coverage/src/main.rs#L140-L141

After 1.0, we should go through everything here and plan FFI for them, filing followups as needed.