x/crypto/cryptotest accepted to test cipher interface quirks

The accepted proposal for x/crypto/cryptotest is the substantive move of the day: a dedicated package to exercise the aliasing, length and state requirements that make crypto/cipher interfaces so easy to implement wrongly. Elsewhere, a write-up on hooking into the Go toolchain via go build -toolexec covers using it for everything from a stopwatch to OpenTelemetry instrumentation, and someone has reimplemented the Total Annihilation engine in Go on Ebitengine with a browser demo, which is a reasonable way to spend a few weekends.