More refactoring for language-independent domain support.

* Renamed "desc"ription unit to "object" wherever possible.
* Added standard x-ref types to a StandardDomain which is always consulted.
* Split domains module into a subpackage.
* Removed additional_xref_types in favor of new directive classes in StandardDomain.
* Implemented x-ref inventory version 2, for all object types.
* Added env.doc_read_data which is for temporary data stored while reading.
* Minimally updated extension tutorial.
* Started to implement changes to interactive search.
* Test suite passes again.

tests/root/objects.txt

@@ -0,0 +1,91 @@
+Testing description units
+=========================
+
+.. function:: func_without_module(a, b, *c[, d])
+
+   Does something.
+
+.. function:: func_without_body()
+
+.. function:: func_noindex
+   :noindex:
+
+
+.. module:: mod
+   :synopsis: Module synopsis.
+   :platform: UNIX
+
+.. function:: func_in_module
+
+.. class:: Cls
+
+   .. method:: meth1
+
+   .. staticmethod:: meths
+
+   .. attribute:: attr
+
+.. explicit class given
+.. method:: Cls.meth2
+
+.. explicit module given
+.. exception:: Error(arg1, arg2)
+   :module: errmod
+
+.. data:: var
+
+
+.. currentmodule:: None
+
+.. function:: func_without_module2() -> annotation
+
+
+C items
+=======
+
+.. c:function:: Sphinx_DoSomething()
+
+.. c:member:: SphinxStruct.member
+
+.. c:macro:: SPHINX_USE_PYTHON
+
+.. c:type:: SphinxType
+
+.. c:var:: sphinx_global
+
+
+References
+==========
+
+:c:func:`CFunction`. :c:func:`!malloc`.
+
+
+Others
+======
+
+.. envvar:: HOME
+
+.. program:: python
+
+.. cmdoption:: -c command
+
+.. program:: perl
+
+.. cmdoption:: -c
+
+
+Testing references
+==================
+
+Referencing :class:`mod.Cls` or :Class:`mod.Cls` should be the same.
+
+
+User markup
+===========
+
+.. userdesc:: myobj:parameter
+
+   Description of userdesc.
+
+
+Referencing :userdescrole:`myobj`.