Init vector state in coverage tester

[ncough]

Sets the vector register array _Z to a series of random values. To simplify this change, also adds a call into the state to randomise this array, along with the existing _R array. Identifies and fixes three issues as a result of new behaviour:

• X[slice] = ... where X is an integer was not supported
• Adds UnsignedRSqrtEstimate and FPRSqrtEstimate to impure prims
• Makes a guess at the implementation of sqrt_real
• Offline copy prop could eliminate an impure operation by propagating it into a lift-time branch. Stop it from propagating any impure operation.