Simplify deposits

[JustinDrake]

Remove unnecessary index field from Deposit while retaining the requirement that deposits are processed in order.

[ralexstokes]

if we merge this we should remove the index from the log in the deposit contract...

[djrtwo]

if we merge this we should remove the index from the log in the deposit contract...

Maybe? The client still needs to hold a local tree of all deposits so the index might be slightly useful?

[djrtwo]

lgtm. I fixed the failing deposit test, but circleci is currently broken.
Let's hold off merge until that's fixed

[ralexstokes]

Maybe? The client still needs to hold a local tree of all deposits so the index might be slightly useful?

@djrtwo i guess this comes down to how strongly we want to have the index be part of the protocol -- if we take it out, clients will likely still have the index locally, it just won't be required for the deposit contract