Here we introduce the `Backtraces` type and associated machinery for
attaching these via `ExceptionContext`. These has a few compile-time
regressions (`T15703` and `T9872d`) due to the additional dependencies
in the exception machinery.

As well, there is a surprisingly large regression in the
`size_hello_artifact` test. This appears to be due to various `Integer` and
`Read` bits now being reachable at link-time. I believe it should be
possible to avoid this but I have accepted the change for now to get the
feature merged.

CLC Proposal: https://github.com/haskell/core-libraries-committee/issues/199
GHC Proposal: https://github.com/ghc-proposals/ghc-proposals/pull/330

Metric Increase:
    T15703
    T9872d
    size_hello_artifact