Encode and decode tag types in dwarf properly. Add list module to std. Shift rustc to use std.util.option. Fix various dependent bugs. Closes #73.

src/comp/util/common.rs

@@ -5,14 +5,6 @@ type pos = rec(uint line, uint col);
 type span = rec(str filename, pos lo, pos hi);
 type spanned[T] = rec(T node, span span);
 
-// FIXME: import std.util.option and use it here.
-// import std.util.option;
-
-tag option[T] {
-  none;
-  some(T);
-}
-
 tag ty_mach {
     ty_i8;
     ty_i16;