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/globalvar.rs

@@ -0,0 +1,37 @@
+//! Global variables.
+
+use ir::GlobalVar;
+use ir::immediates::Offset32;
+use std::fmt;
+
+/// Information about a global variable declaration.
+#[derive(Clone)]
+pub enum GlobalVarData {
+    /// Variable is part of the VM context struct, it's address is a constant offset from the VM
+    /// context pointer.
+    VmCtx {
+        /// Offset from the `vmctx` pointer to this global.
+        offset: Offset32,
+    },
+
+    /// Variable is part of a struct pointed to by another global variable.
+    ///
+    /// The `base` global variable is assumed to contain a pointer to a struct. This global
+    /// variable lives at an offset into the struct.
+    Deref {
+        /// The base pointer global variable.
+        base: GlobalVar,
+
+        /// Byte offset to be added to the pointer loaded from `base`.
+        offset: Offset32,
+    },
+}
+
+impl fmt::Display for GlobalVarData {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &GlobalVarData::VmCtx { offset } => write!(f, "vmctx{}", offset),
+            &GlobalVarData::Deref { base, offset } => write!(f, "deref({}){}", base, offset),
+        }
+    }
+}