Add deposit up to limit functionality

[dyodx]

Closes #274. All feedback welcome.

A couple thoughts:

1. Here the constraint total_deposits_amount < deposit_limit is enforced, so I currently have it going up to deposit_limit - 1.
2. Is there any reason not to have an early return if the deposit amount == 0?

[jgur-psyops]

re (2), there's no reason, we could early return for 0 deposits (or error) to save some compute.

[dyodx]

re (2), there's no reason, we could early return for 0 deposits (or error) to save some compute.

done, i had to rearrange the test because when we return early, a bank account doesn't get created so there are no balances to check. So that seems fine but yea, the concern was because I just don't know what assumptions (or desirable side effects) might be being made somewhere else in this or other systems. One that I want to double check -- would we want to emit LendingAccountDepositEvent even when we return early?

[jgur-psyops]

Consolidated into #295