New features: CodeWriter, TreeFragment, and a transform unit test framework.

New features: CodeWriter, TreeFragment, and a transform unit test framework.

See the documentation of each class for details.

It is a rather big commit, however seperating it is non-trivial. The tests
for all of these features all rely on using each other, so there's a
circular dependency in the tests and I wanted to commit the tests and
features at the same time. (However, the non-test-code does not have a circular
dependency.)