[wip]: Step curve

[mradkov]

Work in progress implementation of a step bonding curve.

Differences:

• alpha() is now used for step size instead of the angle for a linear curve.
• buy_curve() uses Frac.ceil so it can calculate the amount needed for buying x tokens based on the current total_supply
• sell_curve() uses Frac.floor for calculating the sell return

[thepiwo]

tests would be good