Compiler crash when tunnel stitch accesses knot-level temp variable

The compiler crashes when a temp variable created within a knot is accessed from a stitch in that knot, only if the stitch is a tunnel.

The following script results in "System.Exception: RUNTIME ERROR: Variable 'solution' could not be found in context '-1'. This shouldn't be possible so is a bug in the ink engine."

```
=== Ink_Compiler_Crash_Course ===

With gouts of plasma spewing from the left thruster, the starship Ink Compiler was hurled into the planet's atmosphere. Flames ripped across the hull as the surface came closer.

~ temp solution = "Activate the weft drive"

-> save_the_ship -> END

= save_the_ship

Captain Loom knew there was just one chance to save the ship.

"{ solution }," he cried! // knot level temp variable should be accessible from the stitch

->->
```