index_map and character_map are broken (one static map is initialized for all types, and thus doesn't work for different types)

[Trolldemorted]

As discussed in ferrilab/bitvec#229 (comment), the current index_map and character_map functions are broken:

rasn/src/types/strings/constrained.rs

Lines 99 to 112 in fb6e682

	fn index_map() -> &'static alloc::collections::BTreeMap<u32, u32> {
	static MAP: OnceBox<BTreeMap<u32, u32>> = OnceBox::new();
	MAP.get_or_init(|| {
	Box::new(
	Self::CHARACTER_SET
	.iter()
	.copied()
	.enumerate()
	.map(|(i, e)| (e, i as u32))
	.collect(),
	)
	})
	}

The static MAP: OnceBox<BTreeMap<u32, u32>> exists once for all implementing types, not separately for each implementing type.