Reduce accessibility modifiers as much as possible

[ralexstokes]

Some functions in the deposit contract are marked @public when they have no reason to be (other than implementation details of the vyper stack, i.e. they are easier to test when marked @public).

I suggest in the spirit of keeping this contract as simple and small as possible, we reduce the size of the public API by remove the @public decorators where we can.

At the moment that is:

@public
@constant
def to_little_endian_64(value: uint256) -> bytes[8]:

assuming we also address #34.

[JustinDrake]

As you mention, removing @public on to_little_endian_64 breaks the tests.

Please open a PR ASAP if you feel strongly about this :) It will be harder to make changes after spec freeze, and after formal verification of the deposit contract.

[JustinDrake]

Fixed in #1152 :)