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.

lib/cretonne/meta/base/entities.py

@@ -16,6 +16,9 @@ ebb = EntityRefKind(
 #: A reference to a stack slot declared in the function preamble.
 stack_slot = EntityRefKind('stack_slot', 'A stack slot.')
 
+#: A reference to a global variable.
+global_var = EntityRefKind('global_var', 'A global variable.')
+
 #: A reference to a function sugnature declared in the function preamble.
 #: Tbis is used to provide the call signature in an indirect call instruction.
 sig_ref = EntityRefKind('sig_ref', 'A function signature.')