Borrowers set their interest rate and Troves are ordered by interest rate

[RickGriff]

Borrowers set their interest rate and Troves are ordered by interest rate

Fullfills part of #40.

• Each trove has a user-chosen interest rate property
• Borrowers set this upon opening their Trove. We convert their chosen APR (yearly interest rate) to a per-time-unit rate (e.g. per-block or per-minute)
• Borrowers can change their interest rate with changeInterestRate
• The sorted list ordering key is this interest rate: whenever a borrower opens Trove or adjusts interest rate, their trove gets reinserted in the ordered list

TODO:

• Composite function which adjusts both interest rate and coll and/or debt?
• Actually calculate and apply the pending interest (add to Trove’s debt, mint BOLD) at Trove touches, batch updates, and SP touches (see issue X)

[bingen]

Closed by #63