Prefab-classes

BREAKING CHANGE:
import hook has been moved to a new `prefab_classes_hook` package outside of the main `prefab_classes` module.

* import hook now loads more quickly as it no longer requires the main module after original compilation
* prefab_classes imports are removed from compiled prefab modules if the imports are not used after compilation.

What's Changed
* JSON performance tests and a slight to_json performance boost.
* Remove support for converters
* Replace the repr function with a triangle bracket description if it is known that the standard output will not eval correctly.

**Full Changelog**: https://github.com/DavidCEllis/PrefabClasses/compare/v0.8.3...v0.9.0

Fixes a bug when creating dynamic classes with some values being sent to post_init where a missing newline in the template would cause a syntax error.

Adds dynamic `build_prefab` function.

* Faster `to_json` function
* No `inspect` import penalty for defining pre/post init in dynamic classes

Adds support for frozen classes.