allow predicate values to participate in view weighting

[mmerickel]

Copied from a paste in #pyramid irc:

@view_config(
    name='example',
    renderer='example_v2.mako',
    request_param=('foo', 'version=2'))
@view_config(
    name='example',
    renderer='example.mako',
    request_param='foo')
def example_view(self):
  return {}

# >>> GET /example
# HTTP 404 as expected, `foo` parameter not provided

# >>> GET /example?foo
# This renders the example.mako template

# >>> GET /example?foo&version=2
# This renders the example.mako template. Would want the example_v2.mako template
# to render, as it fits -all- of its request_params, which provide for a stricter match.

# N.B. Shuffling view_config statements around provides a solution, the following does
# follow my expectations, though relies on order in code, which I'd like to avoid:


@view_config(
    name='example',
    renderer='example.mako',
    request_param='foo')
@view_config(
    name='example',
    renderer='example_v2.mako',
    request_param=('foo', 'version=2'))
def example_view(self):
  return {}

Basically the two views have different values for their request_param predicate but they do not conflict and are ambiguously ordered. This strategy only works if the predicate values are mutually exclusive such that no value in one view is a subset of values in another view.

We should add a new sortable attribute to predicates or use the phash itself to contribute to the weighting such that 2 views with the same predicate but different values can be sorted appropriately.