Refactor "setVaultDebt" into "increaseVaultDebt" and "reduceVaultDebt"

[Paul Razvan Berg]

The current setVaultDebt function expects the caller fyToken to handle the debt amounts in advance. We could make the protocol more ergonomic if we refactored the function into the following subfunctions:

• increaseVaultDebt
• reduceVaultDebt

This way, the fyToken could simply pass the debt amount as a delta.

[Paul Razvan Berg]

Implemented in 6c98356.