(py-compute-indentation): Several changes made to improve navigation

(py-compute-indentation): Several changes made to improve navigation
over and around triple-quoted strings:

    - move the beginning-of-line to above the p-p-s call

    - in the `t' clause of the big cond, where we skip over
      triple-quoted strings, first find out if we're looking at a
      single or TQS, then skip over it in one fell swoop, instead of
      trying to loop over skipage of SQS's.

(py-parse-state): Implement XEmacs only hack to more accurately figure
out whether we're in a string or not.  Can't do this in Emacs because
it lacks the necessary primitive, so we just do it the old (and mostly
accurate, but foolable) way for Emacs.