Utilities for an abstract fixture format.

Created an abstracted fixture format that is not bound to explicit file
identifiers and file positions. The abstracted fixture format records
the log entires of a leaf pages and the structure of the branch pages.
We still use file numbers for branch pages, but we normalize them for
comparison.

See #130.
Utility to convert current fixtures to new format.

Create a utility to convert existing fixtures to the new fixture format.
It references a `json` folder, which I've not checked in, into which
I'd copied all the existing fixtures.

See #130.
Convert all fixtures to new format.

Added leaf pages deletions to the fixture format and all the fixtures
utilities. Ran the conversion on all existing fixtures.

Closes #130.
Merge branch `fixtures` into `master`.