Add API method to query free and total device memory

[stgeke]

• Serial: mallinfo2?
• CUDA: cudaMemGetInfo
• HIP: hipMemGetInfo
• SYCL: ext::intel::info::device::free_memory
• openCL: No universal support.
  • E.g., see this issue on the Intel compute-runtime repo

[kris-rowe]

Clarifying some finer points for future documentation.

Functions exist which will return

1. How much device memory is currently allocated by OCCA. This value is calculated internally by OCCA.
2. The size of the device memory.

The added value here are functions which will return the total device memory allocated (i.e., by all applications/libraries, not only OCCA) and the remaining available device memory using direct calls runtimes used to implement each backend.