swift234

[utils] Move gyb into a .py file

This allows Python or .gyb files to do "import gyb" and use the basic
components of the system.  For example, we could create a Python
function to generate the boilerplate for a Mirror.  That function could
use gyb.parseTemplate() to load an inline template and
gyb.executeTemplate() to generate the code.  Then calls to that function
can be embedded in ${...} in a .gyb file.

Swift SVN r19490