Add ?, .[]?, and .. operators

Make XPath-like `//a/b` recursive structure traversal easier in jq,
which then becomes:

    ..|.a?.b?

The `?` operator suppresses errors about . not being an array or object.
The `..` operator is equivalent to calling the new `recurse_down`
built-in, which in turn is equivalent to

    recurse(.[]?)

Note that `..a` is not supported; neither is `...a`.  That could be add
added, but it doesn't seem worth the trouble of saving the need to type
a '|'.

opcode_list.h

@@ -6,7 +6,9 @@ OP(LOADV, VARIABLE, 1, 1)
 OP(LOADVN, VARIABLE, 1, 1)
 OP(STOREV, VARIABLE, 1, 0)
 OP(INDEX, NONE,     2, 1)
+OP(INDEX_OPT, NONE,     2, 1)
 OP(EACH,  NONE,     1, 1)
+OP(EACH_OPT,  NONE,     1, 1)
 OP(FORK,  BRANCH,   0, 0)
 OP(JUMP,  BRANCH,   0, 0)
 OP(JUMP_F,BRANCH,   1, 0)