Limit unnecessary calls to Type#toString.

Logging revealed a few thousand calls to the often expensive
Type#toString emerging from tailcalls. The error message was
being generated for all methods even though it was only issued
in rare cases (and for the particular tailrec failure which
made the call, extremely rare.)

The remaining boatload of unnecessary Type#toString calls are
much harder to fix due to the design of "AbsTypeError" and the
fact that the compiler approaches mutability like a cat approaches
a loaded gun. See SI-6149.
Merge pull request #2501 from paulp/pr/tailcalls-tostring

Limit unnecessary calls to Type#toString.