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

[Manishearth]

The FFI coverage test has the following section of ignores:

icu4x/ffi/diplomat/ffi_coverage/src/main.rs

Lines 140 to 141 in 90db7c6

	// Stuff that could be exposed over FFI but is not currently planned
	// =========================

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