`py-shell-complete' re-factored

`py-fast-complete'

New boolean `py-return-result-p'
Internally used. When non-nil, return resulting string of `py-execute-...' functions. Imports will use it with nil.

adapt tests
`py-complete-function' provide `py-fast-complete'
fast-process re-write

Enable fast-process for every Python shell
Don't hard-code a buffer name WRT fast-process, rather extend the
known way by labelling "Fast"
`py-fast-completion-delay', new customizable variable
`py-choose-shell' fixed WRT fast-complete

Py-shell needs `py-shell-complete' reading current state,

not `py-fast-complete'