Simplify implementation of `py:with` directive by compiling to a `Suite`, instead of manually breaking up the statement and compiling each part to an `Expression`. Also, the first line of code in a `Suite` is now stored as the "function name" of the bytecode, so that it shows up in tracebacks.

Sample application that demonstrates the use of Genshi templates in web.py
(http://webpy.org/).

Try introducing errors (not just XML well-formedness errors, those are boring)
in the template, and you'll see a nice error screen with pretty good
information about the exact error.