use unified 'children' member of Model instead of decls/functions/rules

Note that this has the following user-visible effects:
  * White space in C code produced by murphi2c has changed slightly; and
  * XML produced by murphi2xml no longer nests declarations, functions and rules
    under their own sections under <model>, but instead just directly under
    <model>.

Github: related to #202 "remove section ordering restriction"