Remove implicit wrapping/unwrapping of completion records

[h2oche]

According to tc39/ecma262#2547,

• Implicit conversion from value to normal completion only happens in the return of following algorithms:
  • AO annotated to return a completion record
  • the Evaluation SDO
  • built-in functions
• Implicit conversion from normal completion to value is now forbidden.
• Every AO should have explicit return step.