Move second_result outside boxed storage.

In instruction formats that have multiple results AND boxed storage,
place the second_result field outside the boxed storage. The 16-byte
instruction format has room for opcode, type, second_result in the first
8 bytes, and the boxed pointer in the last 8 bytes.

This provides a simpler implementation of the second_result() and
second_result_mut() InstructionData methods.

src/libreader/parser.rs

@@ -1089,10 +1089,8 @@ impl<'a> Parser<'a> {
                 InstructionData::TernaryOverflow {
                     opcode: opcode,
                     ty: VOID,
-                    data: Box::new(TernaryOverflowData {
-                        second_result: NO_VALUE,
-                        args: [lhs, rhs, cin],
-                    }),
+                    second_result: NO_VALUE,
+                    data: Box::new(TernaryOverflowData { args: [lhs, rhs, cin] }),
                 }
             }
             InstructionFormat::Jump => {