chrom_ordering from other files

[brentp]

currently, it's required to get chrom_ordering from a .genome (fai) file. But bedder should also support reading it from a bam/sam/cram header or a .dict file.
It can use the current sniffers and get the appropriate info into the HashMap<String, Chromosome> format:

bedder-rs/src/chrom_ordering.rs

Lines 5 to 12 in fe37df3

	#[derive(Debug, PartialEq, Eq, Clone, Copy)]
	pub struct Chromosome {
	pub(crate) index: usize,
	pub(crate) length: Option<usize>,
	}
	/// A genome is a map from chromosome name to index with an optional chromosome length.
	pub fn parse_genome<R>(reader: R) -> io::Result<HashMap<String, Chromosome>>

[brentp]

Should probably also update length to u64 and use u64::MAX if it's not given.