Refactor ConcreteExtensionField

Restructure / modernization.  Removed exceptions that the library does
not bother to catch.  Fixed warnings with integer precision.
Refactor MutableExtensionRegistry
Refactor CodedInputStream
Refactor RingBuffer
Refactor CodedOutputStream
Refactor TextFormat
Refactor WireFormat
Refactor UnknownFieldSet_Builder
Refactor Field
Refactor MutableField
Refactor UnknownFieldSet
Patch: correct int precision and remove warnings

Added expect cast, the compiler will do the right thing with the
intermediary variable.
Refactor AbstractMessage
Remove exception tests

The new version of the library will not be based on exceptions.  They
were never fully implemented and would propagate to user code.
Refactor GeneratedMessage_Builder
Refactor AbstractMessage_Builder
Refactor ExtendableMessage
Refactor ExtendableMessage_Builder
Refactor GeneratedMessage
Refactor ExtensionRegistry