Add global variables to Cretonne IL.

See #144 for discussion.

- Add a new GlobalVar entity type both in Python and Rust.
- Define a UnaryGlobalVar instruction format containing a GlobalVar
  reference.
- Add a globalvar.rs module defining the GlobalVarData with support for
  'vmctx' and 'deref' global variable kinds.

Langref:
    Add a section about global variables and the global_addr
    instruction.

Parser:
    Add support for the UnaryGlobalVar instruction format as well as
    global variable declarations in the preamble.

cranelift/docs/Makefile

@@ -58,7 +58,7 @@ html:
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 
 autohtml: html
-	$(SPHINXABUILD) -z ../lib/cretonne/meta --ignore '.*.sw?' -b html -E $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	$(SPHINXABUILD) -z ../lib/cretonne/meta --ignore '.*' -b html -E $(ALLSPHINXOPTS) $(BUILDDIR)/html
 
 dirhtml:
 	$(SPHINXBUILD) -b dirhtml $(ALLSPHINXOPTS) $(BUILDDIR)/dirhtml