Rework: new convert() API, higher performance, remember preferred unit exactly

[bradenmacdonald]

This PR is a fairly significant refactor that:

• greatly improves performance (it now beats all other libraries on the benchmark)
• adds a new .convert() API so you can convert to your preferred unit while still getting a Quantity object. The previous .getWithUnits() API for conversions did not return a Quantity object, so was less useful.
• To achieve the improved performance, the (internal) concept of "unit hints" has been replaced with "output unit". This means that each Quantity instance remembers what units it was initialized with, and will use those as the default output unit. (Previously only the "hint" of what unit to use was remembered, and this required more computation at each step to determine the actual exact unit to use).
• Removed the unused luminosity and angle dimensions. Just left one unused placeholder dimension for now. You can still add as many custom dimensions as you want.