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/builder.rs

@@ -5,8 +5,8 @@
 
 use ir::types;
 use ir::{InstructionData, DataFlowGraph};
-use ir::{Opcode, Type, Inst, Value, Ebb, JumpTable, SigRef, FuncRef, StackSlot, ValueList,
-         MemFlags};
+use ir::{Opcode, Type, Inst, Value, Ebb, JumpTable, SigRef, FuncRef, StackSlot, GlobalVar,
+         ValueList, MemFlags};
 use ir::immediates::{Imm64, Uimm8, Ieee32, Ieee64, Offset32, Uoffset32};
 use ir::condcodes::{IntCC, FloatCC};
 use isa::RegUnit;