- fixed a bug that did not discard strings with control characters
between 0x10 and 0x1f
- added termination proofs for two important loops
- made get_ref() constexpr

This introduces a clear separation between test data and test
binaries. Test data is moved into test/data, and the test binaries
move into test/src. A new CMake script specific to building the
tests is introduced in /test to slightly clean up the toplevel
one.

As well as tidying things up, this makes the next step trivial...