Store unit and constant data in TOML

[matterhorn103]

This would make the definitions no longer dependent on Python syntax and therefore also usable from Rust.

Will presumably increase import time, which could be a significant disadvantage, but let's see how big of an effect it actually has.

Does also open the door to decreasing memory use by not actually instantiating units/constants at import time but rather deferring creation until first use, and only storing the definitions in memory. But not sure if this would in reality afford any genuine advantage, the memory footprint is not high.

[matterhorn103]

This is a WIP on the toml branch.

Module progress:

Units:

• astro
• base - won't do, base units should be implemented natively
• chemistry
• common
• computing
• imperial
• logarithmic
• partsper
• prefixed - also consider what should be implemented natively
• si - should probably also be implemented natively
• temperatures
• time
• typography
• us

Constants:

• codata2018
• fundamental - again, probably leave as native implementation