Expand the tests

[mordae]

They now cover all combinations that should round-trip safely and all possible combinations of conversions to make sure nothing ever throws an exception.

[soenkehahn]

@mordae: I also merged #22 now, so we should have working CI (hopefully). If you push a new commit to this branch it should run.

[mordae]

I've restructured the tests so that they now:

1. Verify the identities
2. Verify known-safe round-trips (Unicode -> Unicode -> Unicode and Unicode -> Bytes -> Unicode)
3. Verify crash-safety of the remaining conversions

Is this what you had in mind?

[soenkehahn]

Is this what you had in mind?

Not exactly. What I would like to have is further nested subsections in the test-suite (using either describe or context) to group the tests:

• In the context that's currently called "round trip" there's 4 tests for String roundtrips, 4 tests for StrictText roundtrips and so on. I'd like to have subsections grouping those groups of 4 tests for the same type together.
• Similarly for the context "some conversion", I would prefer to add subsections, one for StrictByteString and one for LazyByteString.

Since there's a bit of a combinatorial explosion going on in this package that we have to manage manually, I'd like to keep things as organized as possible.

Let me know if this is not clear, then I can come up with a commit myself.

[soenkehahn]

@mordae: Thanks for pushing again! That's exactly what I meant.

I put another commit into the PR (1bb25f67). Does that one look good to you? If yes, I'll merge.