Fix subtle bug for lazy completion of File and Submodule

The ETag was not stored by FileDirSubmoduleSymLinkUnionConverter
and FirstMatchUnionConverter. Booo.

(I found the bug while implementing search, but I rebased a few commits
so this doesn't make sense in the git history)

Conflicts:

	manage/CodeGeneration/CodeGenerator.py
Duplicate Converters to ReturnValues before independent slimming
Remove previous value from ReturnValues
Remove eTags from Converters
Remove unused return values and converters
Skip ReturnValues for classes and structs
Remove all _receive.ReturnValues
Refactor generator
Return explicit values from methods

Converters were difficult to understand, and would throw
weird _ConversionExceptions in some cases.
Todos
Remove explicit import of _paginated_list
Force explicit testing of .update and .url on all classes
Rename Release.upload_asset to .create_asset
Extract from https://github.com/github/developer.github.com/commit/80e27fbdba34810d06f8c12351457d8a805db672