Make some code suggestions

[pvanheus]

This PR doesn't change the functionality of the code but rather makes some suggestions in line with Rust idioms. The biggest change is in populate_primer_count_hash where let key = if replaces the double-assignment of key and is_some is used at the insert step and then the rewrite of import_primer_sets to reduce use of mut and prefer a more "functional" style to a more "C-like" approach of initialising and then updating a hashmap.

Further down the code identify_primer_sets has an if-chain replaced with a match based on a clippy suggestion. I'm less sure that this is an improvement - it uses match rather than an if-chain, which is more ideomatic for Rust, but the use of Ordering::Equal etc might make things less readable to some users.