decodetree: Move semantic propagation into classes

Create ExcMultiPattern to hold an set of non-overlapping patterns.
The body of build_tree, prop_format become member functions on this
class.  Add minimal member functions to Pattern and MultiPattern
to allow recusion through the tree.

Move the bulk of build_incmulti_pattern to prop_masks and prop_width
in MultiPattern, since we will need this for both kinds of containers.
Only perform prop_width for variablewidth.

Remove global patterns variable, and pass down container object into
parse_file from main.

No functional change in all of this.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>