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/reader/src/lexer.rs

@@ -37,6 +37,7 @@ pub enum Token<'a> {
     Value(Value), // v12, v7
     Ebb(Ebb), // ebb3
     StackSlot(u32), // ss3
+    GlobalVar(u32), // gv3
     JumpTable(u32), // jt2
     FuncRef(u32), // fn2
     SigRef(u32), // sig2
@@ -308,6 +309,7 @@ impl<'a> Lexer<'a> {
             "v" => Value::with_number(number).map(Token::Value),
             "ebb" => Ebb::with_number(number).map(Token::Ebb),
             "ss" => Some(Token::StackSlot(number)),
+            "gv" => Some(Token::GlobalVar(number)),
             "jt" => Some(Token::JumpTable(number)),
             "fn" => Some(Token::FuncRef(number)),
             "sig" => Some(Token::SigRef(number)),