Make fields of BaseDirectories public

[piegamesde]

Closes #44.

I was initially aiming for making with_env public, but then I noticed that the struct has minimal invariants, so its fields can be made public. Also added some documentation to make more clear which variable contains what exactly, as the prefixes make this somewhat non-obvious.

While doing this I also noted a slight inconsistency: get_runtime_directory does not add any prefixes, however the get_XXX_home functions do. Changing this would require a backwards incompatible change however.

[piegamesde]

Note that I marked the struct as non_exhaustive so that additions of new variables will not cause backwards incompatible changes. This comes at the cost of having to bump MSRV (finding which Rust version introduced the feature was surprisingly painful), and not being able to directly construct the type. I think it closes #44 nevertheless, because one can take the default value or one of the constructors and then modify the remaining values as desired.

I do think the idea of exposing with_env still has some merit, but that change can be made later on independently.

[whitequark]

Thanks!