extras: Add meaningful description to BoringSSL errors

[simonjbeaumont]

Motivation

When an error occurs in a BoringSSL call, we typically embed the packed error code in a CryptoKitError.underlyingCoreCryptoError(error: Int32). This packed error code consists of the library and the reason and BoringSSL has functions for unpacking these and getting descriptive messages for them. I often spend my time in a debugger, manually calling these C functions and then looking things up in header files.

Modifications

In _CryptoExtras (not in Crypto) add a (possibly retroactive) conformance to CustomStringConvertible.

Result

If _CryptoExtras is imported, errors will likely be much more informative. For example,

error: "underlyingCoreCryptoError(error: 50331755)"  // before

error: "lib: bignum routines, reason: INPUT_NOT_REDUCED, code: 50331755"  // after

[Lukasa]

I'm not wild about adding a retroactive conformance: we don't have a unique privilege to add this conformance. Can I suggest instead we add a different overload without a conformance?