Now we can re-create OpeningSystemDefine, and is added as a new test
case to test reading/writing string columns. This was actually quite
fun, it turns out that they like aligning the data sections to a 4-byte
boundary for some reason.
Expanding upon the EXH tests of this same data, we can successfully
write it back (hence why it's testdata now!) A lot of the writing logic
is specific for this EXD, but it's a start.
This adds initial support for writing new/modified maps, which are made up of layer groups (LGB) and terrain (TERA). Neither one is really tested yet, but I started checking some simple examples and it seems to work.
We can simplify some of the repository code since there's now two
separate functions for base/expansion repositories. Added tests to make
sure reading doesn't regress like it did before.
