Create Smoother Migration Path for Non Serde Uses

[mitsuhiko]

Latest master no longer depends on serde by default. This means now that all assertion macros that depend on serde are hidden behind features. This also means that many types no longer implement Serialize. For at least the macros it would be sensible to provide a migration path that looks something like this:

Add a hidden feature _legacy_default_serialization. If this is enabled it auto gates the yaml and json based functionality. However if not also the json (or yaml respectively) features are enabled it emits a deprecation warning that the features need to be enabled.

Followup to #255