Dimensioned rewrite

[paholg]

I am opening this pull request for a couple reasons.

First, it is nearly complete, and I feel really good about this new version of dimensioned, but I would appreciate extra eyeballs taking a look.

Second, as tracking for what still needs to be done:

• The make_units macro. I would like to restructure it somewhat to make its use more flexible.
• Unit systems. For the unit systems we have, they need more constants defined and From implemented for conversions. It would also be nice to add more--at least one fully implemented imperial system.
• The macro for making derived units needs some love.
• Unit tests. We have almost none right now.
• Fix the Root issue.
• Make root functions only defined when modulo is zero.
• Impl fmt traits for cgs, mks.
• Add dimensions to macro
• Eliminate fixmes

Documentation:

• Lib doc
• Unit systems
• Conversion example
• Prefixes
• Traits
• Add constant descriptions to unit system doc
• make_units!
• readme
• changelog