Add specific failure typeLink for async exceptions

[SystemFw]

Overview

Adds a ThreadKilledFailure that you can use with tryEval to act on async exceptions specifically.

Implementation notes

Nothing of note

Interesting/controversial decisions

I'm treating all constructors of AsyncException uniformly (ThreadKilled, UserInterrupt, StackOverflow, HeapOverflow).

Test coverage

Tested via the new tests.u machinery. Currently only tested on the interpreter (see below).

Loose ends

• It can only be tested on the interpreter because the JIT has no typeLinks yet. thread-killed-typeLinks-tests.u should be merged back into concurrency-tests.u when the JIT can run it.
• Requires a builtin.merge to get ThreadKilledFailure, probably something to include in the next release of base

[dolio]

Looks good to me. I don't have a strong opinion about treating the other async exceptions the same. It seems fine, since they're effectively killing the thread in a way that should not be recovered from, just for slightly varying reasons.