Minimize use of autos

[swift-kim]

• Use type deduction in a consistent manner.
  • Based on https://google.github.io/styleguide/cppguide.html#Type_deduction.
  • Avoid use of auto whenever possible.
  • Some exceptions:
  • The type can clearly be inferred from r-value (e.g. used with make_unique<>, get_if<>, reinterpret_cast<>, static_pointer_cast<>).
  • Arguments in a lambda expression.
  • Lambda expression assigned to a local variable.
  • Simple iterator.
  • std::pair<> in foreach loop.
• Consistent variable naming.
  • v → element (in foreach loop)
  • it → iter (iterators)
• Remove unnecessary flutter::.