v0.15.0

Changelog

• 181f622 build: add optional gomutants mutation-testing target
• ff739a2 build: switch gomutants target to szhekpisov/gomutants
• 20629f3 lint: add unnecessary-nil-check rule, widen redundant-nil-guard
• 72330b7 lint: void caller nil-guard proofs when the argument is rewritten
• 2cfd328 release: v0.15.0