namespaces: introduce a generic way to map between names and nodes

This patch begins the work to provide a way to register a namespace to handle
'names'. Benefits of this would be,

- improved templating: This would provide {name} which could output any branch,
  bookmark, tag, or any extension registered namespace all without having the
  extension doing any extra work

- improved tab completion: Since this provides a single source of all 'names',
  tab completion would not need to know of each namespace

- changeset lookup: Similar to before, a unified place to get all 'names' will
  allow finding changesets without any extension code having to reimplement
  this

Also, d226fe36e362 has shown us that for internal code which expects a certain
type of method or behavior, we should provide an easy way for extensions to
check this behavior.

mercurial/namespaces.py

@@ -0,0 +1,25 b''
+from mercurial import util
+
+class namespaces(object):
+    """
+    provides an interface to register a generic many-to-many mapping between
+    some (namespaced) names and nodes. The goal here is to control the
+    pollution of jamming things into tags or bookmarks (in extension-land) and
+    to simplify internal bits of mercurial: log output, tab completion, etc.
+
+    More precisely, we define a list of names (the namespace) and  a mapping of
+    names to nodes. This name mapping returns a list of nodes.
+
+    Furthermore, each name mapping will be passed a name to lookup which might
+    not be in its domain. In this case, each method should return an empty list
+    and not raise an error.
+
+    We'll have a dictionary '_names' where each key is a namespace and
+    its value is a dictionary of functions:
+      'namemap': function that takes a name and returns a list of nodes
+    """
+
+    _names_version = 0
+
+    def __init__(self):
+        self._names = util.sortdict()