CI and Testing Improvements

[mtoohey31]

• Mandate performance or notify about regressions.
• Mandate test and benchmark code coverage. 5decdec
• Mandate 100% godoc coverage. ci: enforce godoc-coverage #13
• Switch to testify instead of using custom asserts. 0bb543a
• Add/convert existing tests to fuzz tests d6174ab
• Run CI in a nix devshell

[mtoohey31]

Item one seems pretty hard to accomplish, given that there's no guarantees about what else might be running on the same machine as the workflow runner. I'm going to mark this as closed without completing that item.