Expose ABI.Version as tools SPI.

[grynspan]

This PR exposes the internal ABI.Version protocol as tools SPI. It includes ABI.VersionNumber (which represents a Swift toolchain version in this context) as well as the basic interfaces of ABI.Record, ABI.EncodedTest, and ABI.EncodedEvent (but not individual fields thereof; we need to audit those individually for usability/stability).

A tool can now use the exposed interfaces to handle an event stream that produces JSON records rather than instances of Event:

func handle<V: ABI.Version>(_ recordJSON: UnsafeRawBufferPointer, using _: V.Type) throws {
  let record = try JSONDecoder().decode(ABI.Record<V>.self, from: Data(recordJSON))
  switch record.kind {
  case let .test(test):
    // Test discovered!
  case let .event(event):
    // Event to handle!
  }
}

In practice, the caller probably already knows the version number here, but just in case, here's how you get it dynamically:

func handle(_ recordJSON: UnsafeRawBufferPointer) throws {
  let versionNumber = try ABI.VersionNumber(fromRecordJSON: recordJSON)
  guard let abi = ABI.version(fromVersionNumber: versionNumber) else {
    throw ...
  }
  try handle(recordJSON, using: abi)
}

There's also ABI.Version.eventHandler(forwardingTo:) that automatically handles the conversion from Event to ABI.Record for tools that want a single code path to handle a built-in copy of Swift Testing and package copies that they don't directly link against.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[harlanhaskins]

This is an excellent first step, and very thorough.