Add a descriptor to clear foreign cache

[vaab]

Often, after calculating a method, we know that we invalidated other cached values... A good example would be ORM, an instance of a record in the database needs cached elements and cached complex function dependent on data of the records, but would then gets invalidated when some write function is called.

This is a little bit like a reverse key: it's not anymore the responsibility of the cache to detect invalidation but foreign functions that could clear the cache. Notice that it's maybe a lesser way to tackle invalidated cache in most situation compared to find the correct key lambda, and I'd like to find a better scenario where it's clear that the key function can't handle it in an elegant way.

>>> class A(object):
...    @cache
...    def complex_fun_on_record_data(self):
...        print 'Compute foo()'
...        return 'hello'
...
...    @classmethod
...    @cache
...    def bar(cls):
...        print "Compute bar()"
...        return "bye"
...
...    @clear_cache(foo)
...    @clear_cache(bar)
...    def data_changed(self):
...        pass