Allow constants to be defined in system! or quantity! macro

[iliekturtles]

Allow constants to be defined in system! or quantity! macro

[iliekturtles]

Replying to @billyrieger in #126. I'd love to see a PR for this. If you want to start working through the open questions and put something together I'd be happy to provide assistance and review.

• Constants are definitely in-scope. I think I would prefer to see them under the si module rather than as a separate crate unless there is a noticeable compilation time change. I also think that a feature may be overkill unless there is a compilation time change. I haven't fully thought this through so I'm open to any suggestions.
• I'm not sure the best way to handle support for non-float underlying storage types either. I've been punting on this issue (see #44) for a while now. Ideally the constants would actually be rust const by using <float literal> as V, but that makes it impossible to create constants for the non built-in types (bigint, ratio, ...). Generally num::FromPrimitive::from_f64 is used right now, but trait methods can't be const.
• Also not sure the best location to place the constants. Using the quantity! macro? What about constants that belong to an unnamed quantity? Using the system! macro? Both? In si/mod.rs? In a constants module under si?
• For constants to include I'd go through the NIST list and include the most common. Just including everything isn't a big deal either.
• Use the constant symbol or the full constant name? Sort of leaning towards the full name.

Based on the limited time I spent thinking through the issue while replying below is one possible solution that is const and works for all built-in types. Usage would be uom::si::constants::f32::G.

pub mod constants {
    storage_types! {
        types: PrimInt, Float;
        /// Gravitational constant.
        pub const G: Quantity<uom::si::ISQ<P3, N1, N2, Z0, Z0, Z0, Z0>, uom::si::SI<f64>, f64> = Quantity { dimension: PhantomData, units: PhantomData, value: 6.674e-11 as V, };
    }
}