Reorganize the global value kinds. (#490)

* Reorganize the global value kinds.

This:
 - renames "deref" global values to "load" and gives it a offset that works
   like the "load" instructions' does
 - adds an explicit "iadd_imm" global value kind, which replaces the
   builtin iadd in "vmctx" and "deref" global values.
 - also renames "globalsym" to "symbol"

lib/codegen/src/predicates.rs

@@ -55,7 +55,7 @@ pub fn is_colocated_func(func_ref: ir::FuncRef, func: &ir::Function) -> bool {
 #[allow(dead_code)]
 pub fn is_colocated_data(global_value: ir::GlobalValue, func: &ir::Function) -> bool {
     match func.global_values[global_value] {
-        ir::GlobalValueData::Sym { colocated, .. } => colocated,
+        ir::GlobalValueData::Symbol { colocated, .. } => colocated,
         _ => panic!("is_colocated_data only makes sense for data with symbolic addresses"),
     }
 }