feat: add seed via exception context

[pbrisbin]

When supported (base-4.20), use addExceptionContext to add the seed to
the failure. This ensures it happens, not just with expectation failures
(HUnitFailure), but also with other thrown-exception failures, and
does so without altering the exception type (as annotated-exception
would).

In base lower than 4.20, behavior is unchanged.

Closes #44.

CAVEAT: this doesn't work :(

HUnit-based testing libraries (e.g hspec) have not updated themselves to
use or display exception context in their output. They would need to
incorporate it into formatFailureMessage for expectation failures.

Even if/when they do, this also doesn't even work for its intended
purpose (non-expectation-failure exceptions) because hspec (for
example) uses show or (optionally) displayException, neither of
which include context (which is wild, IMHO).

So, while it would be nice if we could use exception context for this
information, users would still need to update their test runners somehow
to ensure its displayed.

I know the hspec project is not amenable to displaying more
information like this in its output by default, but I think they do
plan to add some sort of --verbose flag. If/when that existed, I
think we could get them to include any exception context in such output,
and we could instruct our uses to use it if they want to see seed.

But is that how we want it to work? Or should we keep this behavior of
stuffing it into strings that we know are always displayed (and just
give up on #44)? I'm leaning this way.

[cdmren]

I've been playing with removing annotated-exception in the curricula project so I understand a bit more what exception context actually is, now, and I guess it's not quite what I thought it was when I suggested doing that here.

There's an option where we instruct users to add something to a top-level SpecHook that would catch graphula's exception information (be that a seed in an exception context attached to an hunit exception, or a custom exception type containing the seed as a field) and re-throw as an hunit exception with information in the failure reason string. There's an argument to be made that making the hook responsible for error formatting is the only reasonable choice hspec gives you.

I'm not sure it's worth it though.

should we keep this behavior of stuffing it into strings

Seems right.

But is there any harm in also adding it as exception context? It seems like that would allow us to address #44 (make the seed programmatically obtainable from any type of exception) without changing either the exception type or the hunit failure reason string.

[pbrisbin]

But is there any harm in also adding it as exception context?

Makes sense. I'll try that.

[pbrisbin]

OK, I've done it that way. I clobbered what was here and did a "make the change easy / make the easy change" method in two commits:

chore: refactor throw vs exception modification

a04535c

This moves the throwIO out of the function that is concretely
HUnitFailure. This is because I'll next be adding something that needs
to occur between those two points.

feat: add seed to exception context, if supported

21dfb5d

If using base > 4.20 we add the graphula seed to the exception
context of any thrown exceptions.

Closes #44 as best we can. The information won't appear anywhere
automatically, but can be pulled out (e.g.) via SpecHook for those
that know (see README). This is a compromise. Give how hspec works
today, any other method that would make the seed more visible for
non-expectation-failure exceptions comes with too many negative
trade-offs.

[cdmren]

The comment above deprecates this section, did you mean to add this here?

[cdmren]

I wonder if the CPP guard here causes more confusion than it alleviates; GraphulaSeed could be defined and exported regardless of the base version.

[cdmren]

Might be worth adding a test, I find Typeable stuff very finicky.