Fix bug when generating single-source instructions

Single-source instructions need their source spec to be in the the top 4
bits of the source/dest spec, and it was incorrectly being set in the
bottom 4 bits. This affected jump instructions, as they would
be reading the instruction at the given address rather than jumping
to it.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/obj/assemble.rs

@@ -226,13 +226,16 @@ impl Assemble for Inst {
                 let mut bytes = Vec::with_capacity(len);
                 bytes.write_u16::<LE>($op).unwrap();
                 let source = $source;
-                let source_encoding = source.source_encoding();
+                let source_encoding = source.source_encoding() << 4;
                 bytes.write_u8(source_encoding).unwrap();
                 bytes.extend(source.assemble(asm)?);
                 assert_eq!(
-                    self.len(), bytes.len(),
+                    self.len(),
+                    bytes.len(),
                     "instruction size mismatch in {} instruction - {:?} produces these bytes {:?}",
-                    stringify!($op), self, bytes
+                    stringify!($op),
+                    self,
+                    bytes
                 );
                 Ok(bytes)
             }};