Migrate remaining ICU4X FFI to accept potentially ill-formed UTF-8

[sffc]

trafficstars

For the general Diplomat discussion: https://github.com/rust-diplomat/diplomat/issues/471

In ICU4X: we need to decide on unicode_bidi behavior. All remaining &str FFIs need to be migrated to DiploamtStr.

[robertbastian]

@younies over to you for ICU4XMeasureUnitParser::parse_measure_unit