feat(combinators): Adds various basic combinators & impls for std types

[grenewode]

• Adds combinators for fallback and map_value
• Adds implementations of VariableMap for slices, arrays, vectors, Boxs, Rcs and Arcs
• Fixes Fallback to Env when variables missing some key #20 See examples/merge-environments.rs

[de-vri-es]

Thanks for the PR! I think adding these is a nice improvement!

I do think I want to re-organize the module layout a bit, to avoid having all these combinators in the crate root. But don't worry about that, I think it's easier to merge as is and then I can go over the module layout.

I still need to do an in-depth review. Will keep you posted :)

[grenewode]

Yeah, I did notice that; it would be nice to have map::fallback, etc. I couldn't figure out a way to manage that without making larger changes though.

[de-vri-es]

Yeah, no worries. I prefer to do that part myself anyway :)

[grenewode]

I did wonder about adding (T0, T1, ...) instead of fallback. I didn't end up going that with that, because it could be ambiguous with other expectations.

[de-vri-es]

I did wonder about adding (T0, T1, ...) instead of fallback. I didn't end up going that with that, because it could be ambiguous with other expectations.

👍 I think a new struct is indeed more clear and easier to discover in the docs :)

[de-vri-es]

Thanks again!