extras: Add EC toolbox abstractions and OPRF(P-384, SHA-384) VOPRF API (

#292)

## Motivation

We would like to provide support for the P384-SHA384 Verifiable Oblivious Pseudorandom Function (VOPRF) as defined in [RFC 9497: VOPRF Protocol](https://www.rfc-editor.org/rfc/rfc9497.html#name-voprf-protocol).

## Modifications

- Add protocols for some "ECToolbox" abstractions: `Group`, `GroupElement`, `GroupScalar`, `HashToGroup`.
- Add implementations backed by BoringSSL.
- Duplicate some utilities from Crypto into _CryptoExtras: hexadecimal bytes and I2OSP.
- Add VOPRF API, rooted at `enum P384._VOPRF`.

## Results

- New internal abstractions that make it easier to bring up new implementations.
- New API for `OPRF(P-384, SHA-384)` in VOPRF mode.

## Tests

- Test vectors for internal `HashToField` implementation.
- Test vectors for internal VOPRF implementation.
- Tests of the VOPRF public API.

Fix WASI platform build issue (#291)

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Refactor and adopt package access to remove symlinked files (#265)

crypto: Equatable and Hashable conformance for CryptoKitError and Cry…

…ptoKitASN1Error (#266)

Update BoringSSL to 6a2ccdcc2ed1d37a43a2183658d2ae61fd5ce208 (#255)

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Convert PrivacyInfo.xcprivacy symlinks to files (#251)

It is expected behavior of the swift-build build system to copy links as-is, so presenting these as symlinks results in broken symlinks being present in the produced resource bundles, which won't work properly.

On Windows, it even causes the build entirely to fail due to a related bug: swiftlang/swift-foundation#845

Convert these to flat files instead.

extras: Provide initializers for RSA keys from RSA numbers as bytes (#…

…247)

Co-authored-by: Cory Benfield <lukasa@apple.com>

build: Revert to using @_implementationOnly for all BoringSSL modules (…

…#246)