Specify DLPack helper C APIs

In the recent discussions scattered everywhere, it appears that some functions should better be just implemented by DLPack so that the downstream libraries do not have to reinvent the wheels. The possibilities include
1. tensor deleter (see the discussion starting https://github.com/dmlc/dlpack/issues/51#issuecomment-849794854)
2. query if CPU-accessible (see https://github.com/dmlc/dlpack/pull/71#issuecomment-849784565)
3. C APIs corresponding to `__dlpack__` and `__dlpack_device__` in the Python Array API standard for handing streams (#65)
4. C API that could be exposed as a new Python attribute `__dlpack_info__` for returning API and ABI versions (and potentially more, see #34, #72)

cc: @tqchen @rgommers @seberg @eric-wieser @kkraus14 @jakirkham @hameerabbasi @vadimkantorov @oleksandr-pavlyk @szha @veritas9872