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/cdsl/operands.py

@@ -59,7 +59,7 @@ VARIABLE_ARGS = OperandKind(
         'variable_args', """
         A variable size list of `value` operands.
 
-        Use this to represent arguemtns passed to a function call, arguments
+        Use this to represent arguments passed to a function call, arguments
         passed to an extended basic block, or a variable number of results
         returned from an instruction.
         """,