Add a generic portage.cache.mappings.slot_dict_class() function which

Add a generic portage.cache.mappings.slot_dict_class() function which
generates mapping classes that behave similar to a dict but store
values as object attributes that are allocated via __slots__. Instances
of these objects have a smaller memory footprint than a normal dict object.
These classes are used to reduce the memory footprint of the dbapi.aux_get()
caches and the Package.metadata attribute.

svn path=/main/trunk/; revision=10790