Update BoringSSL to 53a17f55247101105ae35767d5c5a6c311843a8e (apple#36)

Update BoringSSL to 5298ef99bf2b2d77600b3bb74dd572027bf495be (apple#31)

Clean up HKDF code. (apple#5)

The UnsafePointer constructors are best avoided, let's use scoped
pointers instead.