Reference solution directory was renamed and move to the
always-excluded-from-delivery .meta folder.

Rename class user test to be more similar to the name of the exercise.
Rename the test class to use the name of the class user test.

With exercism/exercism.io@54e1df3, we can now gather exercises into a
subdirectory. By separating the language track files from the
configuration and management files, it is easier to read the project.

- move gradle config under "exercises" too as it would make a mess
  otherwise.

- where files are required from the "starter" (./src/main/java), these
  are now soft-linked into the example source.
- a couple examples were missing import statements.
- the example code for Hamming was failing the test suite.

At this time, these are all of the problems that have example code.

- enable compilation during CI build only for those that currently pass.
  (let's tackle the failing tests+example code one at a time).

For the related discussion see https://github.com/exercism/xpython/pull/98.