Read device ID from type metadata

[glopesdev]

The current implementation of GetDeviceContext and GetPassthroughDeviceContext explicitly receives the device ID as a numerical argument. While functionally sound, this approach makes it harder to generate informative error messages, since there is no way to map the ID back to a device name.

This PR refactors this approach to instead receive the expected device type. The numerical device ID is then extracted directly from the type metadata by searching for a literal field with the name ID. If no such field is found, or a device with an invalid ID is detected, the exception will now contain the type name.

The change also establishes parity between ContextHelper.GetDeviceContext and DeviceInfo.GetDeviceContext as they now both receive types as arguments.

The main caveat of this approach is that now declaring a static class with a const ID field is no longer optional. Since the approach relies on type metadata, the compiler cannot enforce the compliance of device types with this rule at compile-time. This is probably minor as any errors or inconsistencies will immediately throw errors on any test. The most significant impact will be refactoring the name of this field in the long-term, which would have to be done with care.

Fixes #116

[jonnew]

Looks good, much more helpful.