Move ArbitraryPrecisionInteger type to CryptoBoringWrapper module

[simonjbeaumont]

Motivation

Before this PR, the Crypto module had an internal ArbitraryPrecisionInteger type that manages the lifetime of a BoringSSL BIGNUM and exposes several BIGNUM operations. We'd like to make use of this from the _CryptoExtras module and also for code that will run on Darwin platforms.

Modifications

• Move ArbitraryPrecisionInteger to the CryptoBoringWrapper module.
• Remove compilation guards so it's available on all platforms.
• Mark most operations public.
• Provide RawPointer and MutableRawPointer APIs (and hide typed pointer APIs).
• Provide shim in Crypto for typed BIGNUM pointer APIs.
• Move ArbitraryPrecisionIntegerTests to (new) CryptoWrapperTests test target.

There are a number of other ways this could have been achieved but none of them seemed great given the requirements, specifically:

• We require that any import of the CCryptoBoringSSL module is imported using @_implementationOnly.
• We don't want to expose ArbitraryPrecisionInteger (or, of course, BIGNUM) in the public API of Crypto.
• We would like to not remove the existing @inlineable and @usableFromInline annotations on existing APIs that make use of ArbitraryPrecisionInteger, which itself was @usableFromInline before this PR.

Result

ArbitraryPrecisionInteger is now in a shared place that can be used from the _CryptoExtras module.

[simonjbeaumont]

• We require that any import of the CCryptoBoringSSL module is imported using @_implementationOnly.

@Lukasa based on our discussion, we believe that this is not a requirement and that we can actually drop the @_implementationOnly on our imports, which we can front-run in its own, targeted PR?

[simonjbeaumont]

@swift-server-bot test this please

[Lukasa]

Thanks for this Si, much appreciated.