all: move dev.boringcrypto into main branch behind GOEXPERIMENT

[rsc]

The dev.boringcrypto branch started out as a bit of an experiment, back in the Go 1.8 time frame. It is clearly here to stay as something that we maintain alongside the main distribution.

Maintaining a whole separate branch is cumbersome, requiring frequent conflict resolution during merges and being just generally painful.

It would be far less upkeep if we kept the boringcrypto code in the main branch behind a GOEXPERIMENT, same as we do for GOEXPERIMENT=fieldtrack. We should do that.

This bug is to track work toward that goal. Generally speaking it will require a little bit of rewriting of parts that we can't reasonably merge and then a bunch of build tags.

[gopherbot]

Change https://go.dev/cl/395815 mentions this issue: dashboard: add linux-amd64-boringcrypto builder

[gopherbot]

Change https://go.dev/cl/395881 mentions this issue: [dev.boringcrypto] all: add boringcrypto build tags

[gopherbot]

Change https://go.dev/cl/395879 mentions this issue: [dev.boringcrypto] make.bash: disable GOEXPERIMENT when using bootstrap toolchain

[gopherbot]

Change https://go.dev/cl/395880 mentions this issue: [dev.boringcrypto] internal/goexperiment: add GOEXPERIMENT=boringcrypto

[gopherbot]

Change https://go.dev/cl/395883 mentions this issue: [dev.boringcrypto] crypto/ecdsa, crypto/rsa: use boring.Cache

[gopherbot]

Change https://go.dev/cl/395884 mentions this issue: [dev.boringcrypto] cmd/compile: remove the awful boringcrypto kludge

[gopherbot]

Change https://go.dev/cl/395876 mentions this issue: [dev.boringcrypto] crypto/internal/boring: make SHA calls allocation-free

[gopherbot]

Change https://go.dev/cl/395878 mentions this issue: [dev.boringcrypto] crypto/x509: rename VerifyOptions.IsBoring to AllowCert

[gopherbot]

Change https://go.dev/cl/395882 mentions this issue: [dev.boringcrypto] crypto/internal/boring: add GC-aware cache

[gopherbot]

Change https://go.dev/cl/395877 mentions this issue: [dev.boringcrypto] crypto/..., go/build: align deps test with standard rules

[qmuntal]

@rsc have you considered moving crypto/internal/boring to golang.org/x/crypto/boring and use it as a vendored package? This approach is showing good results in our out-of-tree OpenSSL port, reducing code bloat in the main repo and making it easier to maintain and backport.