Update device ID selection for HIP/CUDA/MAGMA backends

[nbeams]

This PR fixes the device ID selection for the CUDA/HIP backends while retaining the option to use /gpu/cuda or /gpu/hip as shortcuts for the best backends (currently chooses gen). It also adds the ability to set the device ID for the MAGMA backends.

The device ID string was chosen to match that of the OCCA backends, i.e., :device_id=#. An example full resource string would be /gpu/hip/shared:device_id=1.

Other minor changes include:

• Change some headers to fix compiler warnings
• Remove the backend data structs for the shared and gen backends, and have those backends use the base Ceed_Hip or Ceed_Cuda structs. (Previously, the shared and gen structs only contained a Ceed_Hip or Ceed_Cuda pointer called base, but nothing was ever accessed through base and the CUDA/HIP Init functions only ever assumed they were given the base struct directly.)

Resolves #709

[jeremylt]

One of these days we'll have to decide how we want this reaching over into other backend's headers to work.

[nbeams]

@jeremylt Do you think the Rust test failure is the same as what you were talking about on #739?

[jeremylt]

Yes, there is a bug in the nightly builds of Rust, but the nightly builds are required for our current way of doing code coverage in Rust.