WIP: determine tree schema and print it out

[lutter]

This uses the structure of lenses, simplifies it a little bit and adds a
command 'schema' in augtool that makes it possible to print out the schema
for the tree that this lens will produce.

The output format is pretty inscrutable right now and looks like this (for schema Hosts.lns):

*
  |
    { <NULL> = <NULL> }
    { /#comment/ = /[^\001-\004\t\n\r ][^\001-\004\n]*[^\001-\004\t\n\r ]|[^\001-\004\t\n\r ]/ }
    [ /[0-9]+/ = <NULL> ]
      .
        ()
        ()
        { /ipaddr/ = /[^\001-\004\t\n #]+/ }
        ()
        { /canonical/ = /[^\001-\004\t\n #]+/ }
        *
          { /alias/ = /[^\001-\004\t\n #]+/ }
        { /#comment/ = /[^\001-\004\t\n\r ][^\001-\004\n]*[^\001-\004\t\n\r ]|[^\001-\004\t\n\r ]/ }

For operators like *, |, or ., the operands are indented, i.e., the topmost union has three branches. The () means there's a lens that won't put anything in the tree (that output could just be suppressed). For the subtree lens [ l ], the output is either [ key_rx = value_rx ] followed by the (indented) schema of the schema for l, or { key_rx = value_rx } if we know that [ l ] will only produce leaves in the tree.