Add chaining behavior to J and Y

J and Y and now be chained (like YYYYYY) to copy a glyph across the
'wire' they form.

This is a simple implementation of the feature. This changes both J and Y to
have loops in their definitions, instead of being single reads and
writes, which will make them heavier and have a adverse effect on
benchmarks. It also makes it harder to explain how orca's VM works,
since these operators are now non-trivial and can't be used as examples
of trivial operators. But we've decided it's worth it.

sim.c

@@ -499,8 +499,15 @@ END_OPERATOR
 BEGIN_OPERATOR(jump)
   LOWERCASE_REQUIRES_BANG;
   PORT(-1, 0, IN);
-  PORT(1, 0, OUT);
-  POKE(1, 0, PEEK(-1, 0));
+  Glyph g = PEEK(-1, 0);
+  for (Isz i = 1;; ++i) {
+    if (PEEK(i, 0) != This_oper_char) {
+      PORT(i, 0, OUT);
+      POKE(i, 0, g);
+      break;
+    }
+    STUN(i, 0);
+  }
 END_OPERATOR
 
 // Note: this is merged from a pull request without being fully tested or
@@ -700,8 +707,15 @@ END_OPERATOR
 BEGIN_OPERATOR(yump)
   LOWERCASE_REQUIRES_BANG;
   PORT(0, -1, IN);
-  PORT(0, 1, OUT);
-  POKE(0, 1, PEEK(0, -1));
+  Glyph g = PEEK(0, -1);
+  for (Isz i = 1;; ++i) {
+    if (PEEK(0, i) != This_oper_char) {
+      PORT(0, i, OUT);
+      POKE(0, i, g);
+      break;
+    }
+    STUN(0, i);
+  }
 END_OPERATOR
 
 BEGIN_OPERATOR(lerp)