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/mod.rs

@@ -13,17 +13,19 @@ pub mod function;
 mod builder;
 mod extfunc;
 mod funcname;
+mod globalvar;
 mod memflags;
 mod progpoint;
 mod valueloc;
 
 pub use ir::builder::{InstBuilder, InstBuilderBase, InstInserterBase, InsertBuilder};
 pub use ir::dfg::{DataFlowGraph, ValueDef};
-pub use ir::entities::{Ebb, Inst, Value, StackSlot, JumpTable, FuncRef, SigRef};
+pub use ir::entities::{Ebb, Inst, Value, StackSlot, GlobalVar, JumpTable, FuncRef, SigRef};
 pub use ir::extfunc::{Signature, CallConv, ArgumentType, ArgumentExtension, ArgumentPurpose,
                       ExtFuncData};
 pub use ir::funcname::FunctionName;
 pub use ir::function::Function;
+pub use ir::globalvar::GlobalVarData;
 pub use ir::instructions::{Opcode, InstructionData, VariableArgs, ValueList, ValueListPool};
 pub use ir::jumptable::JumpTableData;
 pub use ir::layout::{Layout, CursorBase, Cursor};
@@ -48,3 +50,6 @@ pub type InstEncodings = EntityMap<Inst, isa::Encoding>;
 
 /// Code offsets for EBBs.
 pub type EbbOffsets = EntityMap<Ebb, binemit::CodeOffset>;
+
+/// Map of global variables.
+pub type GlobalVars = EntityMap<GlobalVar, GlobalVarData>;