Add SerializedValue.hasTag replacing MessageSerialization.makeSimpleName

[havocp]

Instead of duplicating the type discriminator out-of-band, add a way to test the usual type discriminator. This is obviously not compatible with duck typing.

We don't want to just use SerializedValue.parse[T] because failed matches would incur exception-throwing overhead, and because we really don't want duck typing in this context because we might accidentally match the wrong kind of event.

[jsuereth]

a few minor comments, but I think help clean up serializedvalue a bit.

[havocp]

It turns out that for LazyValue, we want to compare tag.key not tag (to avoid Type.equals and Mirror.equals), and we can only do it as an optimization because SPickler[Fruit] can be the pickler and Unpickler[Apple] the unpickler.

[jsuereth]

@havocp Yeah, that's right, tag.key not tag. As a side note, my next Yak to shave is reworking how FastTypeTag works in pickling to avoid concurrency bugs in construction.

[jsuereth]

This LGTM,and tests pass.