Improve @u_str implementation to fix downstream precompilation

[MilesCranmer]

Fixes #105 which reported precompilation issues.

• Manual parser for u_str to avoid calling eval (which breaks precompilation)
  • Also implements for us_str
• Creates the NoDims type so that dimension(1.0) doesn't throw a method error.
• Allows you to use uconvert in more contexts, like SymbolicDimensions -> SymbolicDimensions

[MilesCranmer]

@YingboMa what do you think? This completely avoids any eval when evaluating u_str.

I also removed the limitation of type requirement. So you can do things like u"(m, s)".

@gaurav-arya are you okay with this? I think we might have to remove that type requirement to fix precompilation working in downstream packages, since evaluating types requires eval.

[MilesCranmer]

Since this is quite a significant change in the parsing I'm going to sit on this PR for a bit in case I think of anything wrong with it.

@gaurav-arya if you could take a look also that would be amazing

[MilesCranmer]

For backwards compatibility we could have as_quantity injected into the output – then one could still use u"1" to create quantities?

It's not 1:1 with Unitful then but at least it preserves the type stability. Like q = u"1"; q *= 1.5u"m" for example.

[MilesCranmer]

Okay I convinced myself the other way. I think u_str should always return the same type no matter what – it's just the safest thing to do. Even though macros are expanded during compilation, I can easily see the user encountering type instabilities because of this aforementioned issue, where u"1" returns a different type from u"m/m".

This is unlike Unitful, but is like the current implementation of DynamicQuantities. So with this change it's no longer breaking.

[MilesCranmer]

Woo! 100% test coverage!

Alright we're good now. I'm merging as it's no longer a breaking change. @gaurav-arya if you did have a comment on the behavior of u_str we can always implement the breaking change on a new version later.

@YingboMa please confirm that precompilation works on v0.11.2 and feel free to add to the stdlib now.