Remove decimal support

[Muximize]

In #1195 @angularsen says:

If we change all 3 quantities to double, we have the potential to clean up a LOT of QuantityValue complexity.

This made me wonder how deep that complexity goes so I decided to experiment, and this is the result.

I must say some of these changes make me a bit sad. A lot of work and some very clever thinking went into supporting multiple numerical types, and I want to acknowledge that. 🙇

Also, I took it as far as possible but that might not be the best outcome, for example we might want to keep deserialization support. This just demonstrates a possible direction we could go in.

[angularsen]

I synced this with release/v6 branch after merging #1195 #1353

Will do a review of this next.

[Muximize]

I rebased this to clean up the commit history, which should aid in reviewing.