This allows for code to run before and after a schema fragment is
added for the first time.
The anticipated use for this is twofold:
1. To do initial setup, e.g., defining global entities.
2. To 'adopt' unmanaged attributes already defined in the store.
This 'pre' would manually alter or retract attributes so that the
transact of the new schema datoms can complete.
For example, if properties :foo/bar and :foo/baz will be unchanged,
but :noo/zob needs to change from a string to an integer, the :none
pre-function can alter the ident, and the :none post-function can
migrate and clean up.
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