Add label parameter to TLS1PRF

[qmuntal]

This PR updates the TLS1PRF signature to accept the PRF label as a separate parameter:

func TLS1PRF(secret, label, seed []byte, keyLen int, h func() hash.Hash) ([]byte, error)`

It is common for TLS PRF functions to accept the label as a separate parameter instead of prepending it to the seed. This better match the crypto/tls PRF signature: https://github.com/golang/go/blob/d5c5808534f0ad97333b1fd5fff81998f44986fe/src/crypto/tls/prf.go#L48

[dagood]

My only thought is: maybe the performance will be different if we append the seed on the Go side rather than call into cgo twice to have OpenSSL do it.

[ueno]

+1

[qmuntal]

My only thought is: maybe the performance will be different if we append the seed on the Go side rather than call into cgo twice to have OpenSSL do it.

If that slowdown is relevant, then we can always implement part of the function in C while still having a better API 😸