gpu: nova-core: num: add functions to safely convert a const value to a smaller type

gpu: nova-core: num: add functions to safely convert a const value to a smaller type

There are times where we need to store a constant value defined as a
larger type (e.g. through a binding) into a smaller type, knowing
that the value will fit. Rust, unfortunately, only provides us with the
`as` operator for that purpose, the use of which is discouraged as it
silently strips data.

Extend the `num` module with functions allowing to perform the
conversion infallibly, at compile time.

Example:

    const FOO_VALUE: u32 = 1;

    // `FOO_VALUE` fits into a `u8`, so the conversion is valid.
    let foo = num::u32_to_u8::<{ FOO_VALUE }>();

We are going to use this feature extensively in Nova.

Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
[acourbot@nvidia.com: fix mistake in example pointed out by Mikko.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Message-ID: <20251110-gsp_boot-v9-3-8ae4058e3c0e@nvidia.com>