Attempting to use the existing FORK_OPT opcode resulted in difficulty
knowing when to pop an error message off the stack and when not to. This
commit makes DESTRUCTURE_ALT a real opcode that is identical to
FORK_OPT, except for never pushing the error message onto the stack when
continuing from an error backtrack.
Some small changes were necessary to the DUP/POP behavior surrounding
destructuring to accomodate this.
It needs to be possible to do something like
getpath($paths[]) += 1
meaning: increment all the paths in . that are listed in $paths[].
In order to do this getpath() needs to update the jq->path and
jq->value_at_path as necessary.
William Langford