transformers no longer ignore UnApply.fun

Second time's the charm. I remember trying to do exactly the same somewhen
around 2.10.0-M4, but then some continuations tests were failing.
Luckily, today everything went smoothly.

Please note that this fix changes the way that SI-5465 manifests itself.
Previously it produced type errors, now it simply crashes the compiler.
Therefore I had to attach the try/catch FatalError clause to invocations
of toolbox methods, so that compiler crashes get caught and translated to
ToolBoxErrors.

Also fixes SI-7871, and that clears the way for implementing quasiquotes
with conventional macros rather than relying on a special case in typer.