Implement literal generation.

* bzrlib/tests/doc_generate/builders/test_texinfo.py:
(TestTextGeneration.test_literal)
(TestTextGeneration.test_literal_block): Basic test for literals.

* bzrlib/doc_generate/writers/texinfo.py:
(TexinfoTranslator.visit_literal_block)
(TexinfoTranslator.depart_literal_block)
(TexinfoTranslator.visit_literal)
(TexinfoTranslator.depart_literal): Support literal words and blocks.