Bug: `HMAC.Algorithm.fromNative` has a duplicate dictionary key, crashes on first access

[alexhunsley]

HMAC.swift's fromNative lookup table maps native CCHmacAlgorithm constants back to HMAC.Algorithm cases:

static let fromNative: [CCHmacAlgorithm: Algorithm] = [
    CCHmacAlgorithm(kCCHmacAlgSHA1): .sha1,
    CCHmacAlgorithm(kCCHmacAlgSHA1): .md5,   // should be kCCHmacAlgMD5
    ...
]

kCCHmacAlgSHA1 is used as the key for both .sha1 and .md5 — the .md5 entry should use kCCHmacAlgMD5. Since Swift dictionary
literals fatal-error on duplicate keys, any access to fromNative (e.g. via fromNativeValue(nativeAlg:)) traps at runtime with
Fatal error: Dictionary literal contains duplicate keys.

Both members are internal, so this isn't reachable by consumers of the library today, and there's no existing test coverage for
fromNativeValue at all — likely why this has gone unnoticed.

Fix: use kCCHmacAlgMD5 for the .md5 entry, plus a regression test that round-trips every Algorithm case through nativeValue() →
fromNativeValue().

[alexhunsley]

Suggested unit test:

	func test_HMAC_Algorithm_fromNativeValue_roundTrips() {

		#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)

			func isSame(_ a: HMAC.Algorithm, _ b: HMAC.Algorithm) -> Bool {
				switch (a, b) {
				case (.md5, .md5), (.sha1, .sha1), (.sha224, .sha224),
					 (.sha256, .sha256), (.sha384, .sha384), (.sha512, .sha512):
					return true
				default:
					return false
				}
			}

			let algorithms: [HMAC.Algorithm] = [.md5, .sha1, .sha224, .sha256, .sha384, .sha512]

			for algorithm in algorithms {

				guard let roundTripped = HMAC.Algorithm.fromNativeValue(nativeAlg: algorithm.nativeValue()) else {
					XCTFail("fromNativeValue(nativeAlg:) returned nil for \(algorithm)")
					continue
				}

				XCTAssertTrue(isSame(algorithm, roundTripped), "\(algorithm) round-tripped to \(roundTripped) instead of itself")
			}

		#endif
	}