Make `memset_s` consistent with Darwin. (apple#52)

Resolves [issue#51](apple#51).

In Swift 5.3, a warning is viewed on Linux because `memset_s` implemented in this module returns `Void`.
This commit let the function return the same type with Darwin.

Update BoringSSL to 67818bea6690a230e2f42e8a588e0f54949bbbf1 (apple#50)

Update BoringSSL to 54858b63c1d886f6c8d903d4a4f594f1485de189 (apple#47)

Update BoringSSL to 54858b63c1d886f6c8d903d4a4f594f1485de189 (apple#47)

Update Swift Crypto with API from CryptoKit @ WWDC20 (apple#40)

Motivation:

At WWDC 2020 Apple updated the CryptoKit APIs. This PR brings those new
APIs to Swift Crypto. This brings Swift Crypto up-to-date with the
latest CryptoKit API support, including PEM/DER key support and HKDF API
extensions.

Modifications:

- Bring forward the CryptoKit API files
- Update to have API match the WWDC seed.

Result:

Users will have access to the seed APIs.

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.