Add support for i128/u128

[iliekturtles]

Add support for i128/u128 now that both types are stable. This may be blocked by the num-* crates which may not support these types. Leave types out of CI build scripts for the moment. A larger effort to better support different rustc versions will need to be done.

[iliekturtles]

Consider adding a build script to detect the rustc version and remove the manual version numbers. This can also be used to control i128/u128 features.

https://www.reddit.com/r/rust/comments/8ioqyk/how_should_a_library_support_u128/

[Aehmlo]

I think we should be good on the other crates.

[Aehmlo]

I think this is was what I was thinking of in #150, for what it's worth.

[iliekturtles]

i128/u128 were stabilized in 1.26.0. Now that uom requires 1.28.0 these can be added without any version checking!