Add PrimitiveNumber::CONST for 0..=127

[cuviper]

These are all of the numbers common to every primitive numeric type,
available in constant form. Each value corresponds to its usize index.

This could be seen as a replacement and extension of ConstZero::ZERO
and ConstOne::ONE from num-traits, without worrying about the
particulars of being additive/multiplicative identities. Here, we're
just describing this as a workaround for the lack of generic literals.

[cuviper]

Naming is hard -- this might be better with a longer name like CONSTANTS, SMALL_VALUES, etc. But I think in use, examples like T::CONST[0] and T::CONST[42] are nice and succinct.

[cuviper]

I'm having second thoughts about this, because the array access doesn't get optimized in the front end like I hoped. In the milu example I put in the docs, with 3 CONST accesses, the LLVM IR ends up with 3 full alloca+memcpy for the whole array, even in release mode (pre-optimization). That does ultimately get optimized out in release mode, but every memcpy remains in the final output in debug mode.

It's better when that's forced to compile time, e.g. const { T::CONST[_] }, but that's more cumbersome. I might see if we can make that easier with a macro, perhaps.