fix non-determinism in FeatureParamsCharacterVariants nameIDs

[anthrotype]

Part of fixing #1568

We were iterating over HashMap's items while incrementally assigning new nameIDs for the Feature Parameters Character Variants, but the iteration order changes each time, so best to use a BTreeMap in this case

fontc/fea-rs/src/compile/features.rs

Lines 256 to 259 in 00c6112

	for (tag, cv_params) in self.character_variants.iter() {
	let params = cv_params.build(name_builder);
	result.insert(*tag, FeatureParams::CharacterVariant(params));
	}

After applying this patch, running the following command multiple times produces the same GSUB table:

python3 resources/scripts/ttx_diff.py 'https://github.com/calcom/font?b833fb1129#sources/Cal-Sans.glyphs'

JMM

[cmyr]

yep, good call :)