Bring AST repr closer to the real transform syntax

[finn-rudolph]

• The representation is always to be understood with the following imports (We don't print them, I think this would be too pedantic):

from pydiverse.transform import Table
from pydiverse.transform.extended import *

• If a table name is not a valid python identifier, AST printing in this form may fail. (We handle spaces, dots and hyphens by replacing them with underscores)
• To get from the printed AST repr to running source code, one needs to
  • fix the table definitions and fill them with actual data
  • Sometimes break the flow and introduce variables for aliases