Redundant `set-default', minor cleanup
`py-statement' no longer aliased to `py-copy-statement'

Same with block, def, expression etc.

`py-statement' made own command, which returns
statement, a string. Otherwise the kill-ring might get
cluttered by function-calls from programs.

Internal function `py-base' renamed `py--base'
`py-mark-base' fixed

py-copy-statement-test added