The only reason we were using qtls.HkdfExpandLabel was to test our own
implementation of HKDF-Expand-Label. By using a pre-generated test
vector, we won't have to expose this function from qtls any more.
The standard library uses cryptobyte.Builder in hkdfExpandLabel. This
costs quite a bit of performance. Using an optimized implementation
speeds up the initialization of the AEAD used for the Initial encryption
level by ~15%.
