message: Apply comments.
  Also adds a bit more of opcodes.
diff: use assert_matches::assert_matches;
  use parity_wasm::{
  builder,
  - elements::{Instruction, Instructions},
  + elements::{BlockType, Instruction, Instructions},
  };
  use casper_engine_test_support::{
  @@ -137,6 +137,12 @@ fn should_correctly_measure_gas_for_opcodes() {
  const GROW_PAGES: u32 = 1;
  + // A vector of expected cost of given WASM instruction.
  + // First element of the tuple represents and option where Some case represents metered
  + // instruction and None an instruction that's not accounted for.
  + //
  + // The idea here is to execute hand written WASM and compare the execution result's gas counter
  + // with the expected gathered from here.
  let opcodes = vec![
  (Some(opcode_costs.nop), Instruction::Nop),
  (
  @@ -170,6 +176,28 @@ fn should_correctly_measure_gas_for_opcodes() {
  (Some(opcode_costs.op_const), Instruction::I32Const(0)),
  (Some(opcode_costs.store), Instruction::I32Store(0, 4)), /* Store `grow_memory` result
  * whatever it is */
  + // if 0 { nop } else { nop; nop; }
  + (Some(opcode_costs.op_const), Instruction::I32Const(0)),
  + (
  + Some(opcode_costs.control_flow),
  + Instruction::If(BlockType::NoResult),
  + ),
  + (None, Instruction::Nop),
  + (None, Instruction::Else),
  + // else clause is accounted for only
  + (Some(opcode_costs.nop), Instruction::Nop),
  + (Some(opcode_costs.nop), Instruction::Nop),
  + (None, Instruction::End),
  + // 0 == 1
  + (Some(opcode_costs.op_const), Instruction::I32Const(0)),
  + (Some(opcode_costs.op_const), Instruction::I32Const(1)),
  + (Some(opcode_costs.integer_comparsion), Instruction::I32Eqz),
  + (Some(opcode_costs.store), Instruction::I32Store(0, 4)), /* Store `eqz` result
  + * whatever it is */
  + // i32 -> i64
  + (Some(opcode_costs.op_const), Instruction::I32Const(123)),
  + (Some(opcode_costs.conversion), Instruction::I64ExtendSI32),
  + (Some(opcode_costs.control_flow), Instruction::Drop), /* Discard the result */
  // Sentinel instruction that's required to be present but it's not accounted for
  (None, Instruction::End),
  ];
  