add rust sig building utilities

[bluegenes]

Over in sourmash-bio/sourmash_plugin_branchwater#516, I added a series of utilities for building and writing signatures in rust. These are an improvement and refactoring of the code I initially wrote for branchwater manysketch and some code I wrote for directsketch.

Refactoring and further development of BuildUtils mostly happened across these PRs:

• MRG: refactor sketching utilities sourmash_plugin_directsketch#112
• MRG: validate param strings sourmash_plugin_directsketch#114
• MRG: remove BuildParams, filter via manifest / Select approaches sourmash_plugin_directsketch#127
• MRG: integrate sketching utils from directsketch sourmash_plugin_branchwater#516

My intention with these utils is to add them to sourmash core once we get them to a good spot, which means that the license for this code will change to BSD.