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/src/ir/instructions.rs

@@ -109,6 +109,10 @@ pub enum InstructionData {
     UnaryIeee32 { opcode: Opcode, imm: Ieee32 },
     UnaryIeee64 { opcode: Opcode, imm: Ieee64 },
     UnaryBool { opcode: Opcode, imm: bool },
+    UnaryGlobalVar {
+        opcode: Opcode,
+        global_var: ir::GlobalVar,
+    },
     Binary { opcode: Opcode, args: [Value; 2] },
     BinaryImm {
         opcode: Opcode,