Audit and test that validation order is consistent

[jswrenn]

Our conversion methods should perform their runtime validation checks in a consistent order; e.g., checking alignment, then size, then bit validity. This ordering should be both internally consistent, and, ideally, SemVer stable.

[jswrenn]

Removed the blocking-next-release This issue should be resolved before we release on crates.io label, in favor of the more moderate approach of documented that we strive for a particular validation order, but we do not actually guarantee it.