* Make C++ error messages more like the Go ones

* Bugfix: field named 'const' has accessor 'const_'

A field named "const" is accessed as "m.const_()", so generate code
appropriately.

* Predeclare Validate() functions to support nesting

* Use correct message name in C++ output

* Add C++ validation library

* Use fully-qualified C++ message class names

* Implement nested validation in C++

This is ugly, because we don't necessarily generate a validation
function for every proto message type (ex: the well-known
google::protobuf::Any type). Use some template metaprogramming to work
around this, where each validation function is invoked if it exists, and
a default fallback version is provided as well.