Rename ArrayContainerT

[majosm]

Closes#80.

I noticed a couple of spots where it makes sense (I think) to require containers, so I also added a ContainerT in addition to ArrayOrContainerT.

[majosm]

I had to remove a couple of the ContainerT usage instances to fix mypy failures, so it's maybe a little less useful than I thought. I don't suppose there's a way to indicate that a ContainerT is also an ArrayOrContainerT? I think that's (at least partially) what's throwing mypy off.

[majosm]

I wonder if I could do something like:

ArrayT = TypeVar("ArrayT")
ContainerT = TypeVar("ContainerT")
ArrayOrContainerT = Union[ArrayT, ContainerT]

?

[majosm]

That seems to work. Is it OK to add ArrayT? (Should I put it somewhere other than arraycontext.container?)

[alexfikl]

I'm definitely on board with the renaming, since it's a lot clearer!

I'm not very convinced the type annotations for the two *_impl functions are correct though, mostly because they nest generic functions and reuse the same TypeVar. There are some rules about that in PEP 484, but they don't seem to mention this setup.

This was definitely an issue before this PR, so not really a showstopper, just thinking out loud.

[inducer]

LGTM. Thanks! I'll wait with merging until the CI issue from inducer/pytato#145 is done.