bellpepper-emulated: Avoid AssignmentMissing error when using MetricCS

[avras]

The changes in this PR are the following:

• Add a scalar multiplication test in ed25519 that uses MetricCS
• Avoid returning SynthesisError::AssignmentMissing in alloc_num_equals_constant
• Avoid returning SynthesisError::AssignmentMissing in conditionally_select

ed25519 scalar multiplication using MetricCS fails with an AssignmentMissing error.

In emulated/src/util.rs:alloc_num_equals_constant, the function was returning SynthesisError::AssignmentMissing when used with MetricCS. The input a returned None for a.get_value(). An emulated library user also reported synthesis errors at this juncture.

After fixing the above, the AssignmentMissing error occurred in emulated/src/field_element.rs:conditionally_select. The below comment does not hold if condition is set to a Boolean::Constant. So condition could have a value when used with MetricCS but the limbs may not have values.

  // If the condition has a value, then the limbs must also have a value, so we bubble
  // the assignment missing error in this case

For example, in ed25519 scalar multiplication, the bits corresponding to the scalar can be constant Booleans. It is possible a fixed group element is being calculated in the circuit.