DRAFT: redesign - track cumulative borrows using a u64 counter instead of a bool flag

[billythedummy]

Suggested Modifications:

• track borrows by saving the current borrowed amount in the pool account instead of using a boolean flag
• remove amount arg from repay; the instruction just repays the current borrowed amount saved in the pool account

This should enable safe CPI borrows and repays, including multiple borrows in the same tx. I haven't fully thought through if this change opens new vectors of attack but reducing user input should reduce attack surface.

Just a suggestion, feel free to disregard