feat: allow subsets of dtypes, allow device-dependent dtype support

[ev-br]

Per discussion in gh-998 and the community meeting on Apr 30th,

• soften the requirement that all dtypes are implemented from must to should;
• require that at least one dtype per category is actually supported (this keeps excluding array libraries which do not have complex types at all, but allows us to not bother with edge cases of e.g. no floats at all);
• recommend that unsupported combinations of dtype=..., device=... raise an exception + specify that creation functions should raise. Note that this still leaves astype and to_device only implicitly specified and only covered by the recommendation to raise.
• specify that cross-device transfers in __getitem__ and __setitem__ are implementation-defined (and presumably covered by the recommendation to raise).