use a new struct for symbols

Symbols are a useful abstraction
that occurs in both Con and Alias.
In this patch they get their own
struct. This new struct packages
a symbol name and a type; the type
tells us where the symbol name
must be interpreted (currently, in
gobal memory or in thread-local
storage).

The refactor fixed a bug in
addcon(), proving the value of
packaging symbol names with their
type.

load.c

@@ -152,7 +152,9 @@ load(Slice sl, bits msk, Loc *l)
 			break;
 		case ACon:
 		case ASym:
-			c = curf->con[a->u.sym.con];
+			memset(&c, 0, sizeof c);
+			c.type = CAddr;
+			c.sym = a->u.sym;
 			c.bits.i = a->offset;
 			r = newcon(&c, curf);
 			break;