Simplify deposits

[JustinDrake]

Fix #760

[hwwhww]

LGTM 👍
pending on CI to verify.

[djrtwo]

This PR requires two things:

1. a corresponding change in the deposit contract repo
2. an assertion in the deposit contract that ensures that the amount contained in thedeposit_data payload is equivalent to the amount of eth that came in on the tx. Otherwise, a user could send a 1eth tx but put in the deposit_data.amount some value higher than 1

[JustinDrake]

Opened an issue :)

ethereum/deposit_contract#24

[NIC619]

Opened the PR at
ethereum/deposit_contract#25 .

I didn't change much: first compute amount from msg.value then concatenate it with deposit_input.
But this differs from the spec in the order of the data in deposit_data:
this PR has amount + pubkey + withdrawal_credentials + proof_of_possession,
spec has pubkey + withdrawal_credentials + amount + proof_of_possession.
I can change the order if the spec one is preferred.

[JustinDrake]

@NIC619 The spec ordering was chosen to be consistent with the Validator object which has pubkey and withdrawal_credentials as the first two fields (those are the two fixed fields).

[NIC619]

@JustinDrake Updated in
ethereum/deposit_contract@4c8ad52

[djrtwo]

Similar to #779 , @NIC619 I think we should merge ethereum/deposit_contract#25 into dev only to be released when this is released via v0.6.0 as to not confuse those targeting v0.5.x

[djrtwo]

Looks good. Added tests, fixed a bug.