Use consistent naming for {Composing,Decomposing}Normalizer FFI

[linusg]

DecomposingNormalizer's is_normalized{,_up_to} is the odd one out:

$ git grep is_normalized ffi/capi/bindings/c/
ffi/capi/bindings/c/ComposingNormalizer.h:bool icu4x_ComposingNormalizer_is_normalized_utf8_mv1(const ComposingNormalizer* self, DiplomatStringView s);
ffi/capi/bindings/c/ComposingNormalizer.h:bool icu4x_ComposingNormalizer_is_normalized_utf16_mv1(const ComposingNormalizer* self, DiplomatString16View s);
ffi/capi/bindings/c/ComposingNormalizer.h:size_t icu4x_ComposingNormalizer_is_normalized_utf8_up_to_mv1(const ComposingNormalizer* self, DiplomatStringView s);
ffi/capi/bindings/c/ComposingNormalizer.h:size_t icu4x_ComposingNormalizer_is_normalized_utf16_up_to_mv1(const ComposingNormalizer* self, DiplomatString16View s);
ffi/capi/bindings/c/DecomposingNormalizer.h:bool icu4x_DecomposingNormalizer_is_normalized_mv1(const DecomposingNormalizer* self, DiplomatStringView s);
ffi/capi/bindings/c/DecomposingNormalizer.h:bool icu4x_DecomposingNormalizer_is_normalized_utf16_mv1(const DecomposingNormalizer* self, DiplomatString16View s);
ffi/capi/bindings/c/DecomposingNormalizer.h:size_t icu4x_DecomposingNormalizer_is_normalized_up_to_mv1(const DecomposingNormalizer* self, DiplomatStringView s);
ffi/capi/bindings/c/DecomposingNormalizer.h:size_t icu4x_DecomposingNormalizer_is_normalized_utf16_up_to_mv1(const DecomposingNormalizer* self, DiplomatString16View s);

[Manishearth]

Unfortunately this is an FFI breaking change. We should stick this in the 3.0 breaking milestone.