Use plum for type dispatch

[RensDimmendaal]

Part of moving dispatch and transform from fastcore to fasttransform (see here.

This PR removes the use of fastcore.dispatch.TypeDispatch and replaces it with plum.Function.

This needs to be done before merging the PR that removes dispatch and transform from fastcore. Because as long as nbdev depends on fastcore.dispatch removing the module will cause fastcore's integration tests to break.

Open questions:

• version bump to which version?
• changelog update?

[RensDimmendaal]

will re-open w cleaned up git history.