Overhaul traits

[casey]

• Remove superfluous traits. I think we should focus on a small number of high value traits. Name and rarity level seem like they pull their weight, the others don't.
• Add a fourth rarity tier. This displaced the old legendary, which is now epic. The fourth tier is the first ordinal in the first block of every conjunction of the difficulty period and the halving epoch.
• Introduce a "degree" trait. Degree is a single trait that allows assessing rarity at a glance. The degree of an ordinals is CYCLE°EPOCH_OFFSET′PERIOD_OFFSET″BLOCK_OFFSET‴. Degrees and ordinals are a bijection, you can calculate the ordinal from the degree, and vice versa.