Spec cleanup - #35

Summary

• Variable/Scope
• Remove member access expression
• Change "imperative language block" -> "imperative block", remove "associative"
• Remove type conversion rules from number/string/etc to bool. Type conversion is stricter so that the following implicit type conversion will not be allowed:

  [Imperative] {
      x = {1, 2, 3};
      while (x) {
          ...
      }
  }
  

Updated spec: https://github.com/ke-yu/DesignScript/blob/update/LanguageSpec.md

@pboyer PTAL