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/instructions.py

@@ -345,6 +345,18 @@ stack_addr = Instruction(
         """,
         ins=(SS, Offset), outs=addr)
 
+#
+# Global variables.
+#
+
+GV = Operand('GV', entities.global_var)
+
+global_addr = Instruction(
+        'global_addr', r"""
+        Compute the address of global variable GV.
+        """,
+        ins=GV, outs=addr)
+
 #
 # WebAssembly bounds-checked heap accesses.
 #