SI-8637 fixes toolbox phase corruption

It turns out, Toolbox.typecheck hasn't been properly re-initializing its state
between requests.

In particular, globalPhase was left untouched, which made the compiler
think that it's past typer, and that in turn disabled implicits.

This commit applies a symptomatic fix to this problem.
Merge pull request #3803 from xeno-by/ticket/8637

SI-8637 fixes toolbox phase corruption