Use int_sqrt

[djrtwo]

Issue

Spec now uses int_sqrt instead of floating point sqrt and then an integer cast.

Proposed Implementation

• implement int_sqrt in helpers

  def int_sqrt(n):
  x = n
  y = (x + 1) // 2
  while y < x:
      x = y
      y = (x + n // x) // 2
  return x

• replace usage of int(sqrt()) in rewards calculations with int_sqrt

[djrtwo]

taking care of this

[djrtwo]

Linking to the two relevant PRs in the spec repo https://github.com/ethereum/eth2.0-specs/pull/13 https://github.com/ethereum/eth2.0-specs/pull/14