Require StreamLabel be 2-byte aligned for serialization

[sharwell]

Forcing StreamLabel to be 2-byte aligned for serialization allows it to be serialized without loss of precision with a right shift of 1. This in turn allows the 4GiB file size limit for .gcdump and .etlx to be raised to 8GiB. This pull request implements the first step of this process.

This is a breaking change to the serialization format of FastSerialization, but it's not clear whether or not that is a problem.