Throw instead of aborting on wire type mismatches in the Swift runtime

[oldergod]

Harden the Swift decoder so that bad bytes throw an error and do not stop the process.

ProtoReader.readBuffer, readData, readFixed32, readFixed64 and readVarint check that the wire type of the current field is the one they read. When the check failed, they called fatalError or precondition. Generated code picks the read primitive from the schema, but the wire type comes from the input. So a known field number with a different wire type stopped the process, in debug and in release builds. A do/catch around ProtoDecoder.decode cannot handle this.

The five functions already throw, so no caller changes. On a mismatch they now throw ProtoDecoder.Error.invalidStructure(message:). This is the same approach as beginMessage, which throws messageWithoutLength for its own state check, and as the Kotlin runtime, which throws ProtocolException. I did not use invalidFieldWireType, because that case reports a wire type value that does not exist.

Valid input, truncated input and unknown fields decode the same as before.

Tests:

• ProtoReaderTests: one test for each of the five primitives. Each test gives a field key with a different wire type and expects invalidStructure with the message.
• ProtoDecoderTests.testDecodeRejectsKnownFieldWithMismatchedWireType: decodes Person with known fields sent with each wrong wire type, and expects a thrown error.

Without the fix, each new test stops the test process with signal 5 at its own check.