Add CodeWriter utility class.

Helps simplify code generation code.  Instead of this:
  code += "inline const " + cpp_qualified_name + " *Get";
  code += name;
  code += "(const void *buf) {\n  return flatbuffers::GetRoot<";
  code += cpp_qualified_name + ">(buf);\n}\n\n";

You do this:
  code.SetValue("NAME", struct_def.name);
  code.SetValue("CPP_NAME", cpp_qualified_name);
  code += "inline const {{CPP_NAME}} *Get{{NAME}}(const void *buf) {";
  code += "  return flatbuffers::GetRoot<{{CPP_NAME}}>(buf);";
  code += "}";
  code += "";

Updated the CPP code generator to use the CodeWriter class.  Most of the
changes in the generated code are white-space changes, esp. around new
lines (since the code generator class automatically appends new lines
when appending a string).  Actual code changes include:

* Renamed "rehasher" to "_rehasher" for consistency with other args in
  Pack function.

* Renamed "union_obj" to "obj: in UnPack function.

* Always do "(void)_o;" to prevent unused variable warning in Create
  function (instead of only doing it if there are no fields) in order
  to avoid two-passes.

* Renamed padding variables from __paddingX to paddingX__.
  "Each name that contains a double underscore (_ _) [...] is reserved
   to the implementation for any use."  C++ standards 17.4.3.1.2.

* Add braces around switch cases.

* Calculate index as a separate statement in EnumName function, eg.
    const size_t index = ...;
    return EnumNamesX()[index];
  vs.
    return EnumNamesX()[...];

* Stored end table offset in variable in Finish() functions, eg.
    const auto end = fbb_.EndTable(start_, ...);
    auto o = flatbuffers::Offset<T>(end);
  vs.
    auto o = flatbuffers::Offset<T>(fbb_.EndTable(start, ...));

* Separate reinterpret_cast calls from function calls in Union
  functions, eg.
    auto ptr = reinterpret_cast<const T *>(obj);
    return ptr->UnPack(resolver);
  vs.
    return reinterpret_cast<const T *>(obj)->UnPack(resolver);

* Removed unecessary (void)(padding__X) no-ops from constructors, eg.
    Test(int16_t a, int8_t b) : ... {
      (void)__padding0;  // <-- Removed this line.
    }

In the idl_gen_cpp.cpp file itself, I refactored some code generation into
new functions: GenParam, GenNativeTable, GenVerifyCall, GenBuilders,
GenUnpackFieldStatement, and GenCreateParam.

Change-Id: I727b1bd8719d05b7ce33cbce00eb58fda817b25d