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/function.rs

@@ -4,8 +4,8 @@
 //! instructions.
 
 use entity_map::{EntityMap, PrimaryEntityData};
-use ir::{FunctionName, CallConv, Signature, JumpTableData, DataFlowGraph, Layout};
-use ir::{JumpTables, InstEncodings, ValueLocations, StackSlots, EbbOffsets};
+use ir::{FunctionName, CallConv, Signature, JumpTableData, GlobalVarData, DataFlowGraph, Layout};
+use ir::{JumpTables, InstEncodings, ValueLocations, StackSlots, GlobalVars, EbbOffsets};
 use isa::TargetIsa;
 use std::fmt;
 use write::write_function;
@@ -25,6 +25,9 @@ pub struct Function {
     /// Stack slots allocated in this function.
     pub stack_slots: StackSlots,
 
+    /// Global variables referenced.
+    pub global_vars: GlobalVars,
+
     /// Jump tables used in this function.
     pub jump_tables: JumpTables,
 
@@ -50,6 +53,7 @@ pub struct Function {
 }
 
 impl PrimaryEntityData for JumpTableData {}
+impl PrimaryEntityData for GlobalVarData {}
 
 impl Function {
     /// Create a function with the given name and signature.
@@ -58,6 +62,7 @@ impl Function {
             name,
             signature: sig,
             stack_slots: StackSlots::new(),
+            global_vars: GlobalVars::new(),
             jump_tables: EntityMap::new(),
             dfg: DataFlowGraph::new(),
             layout: Layout::new(),