Probably good to document this more loudly for folks that try to use
Async Algorithms.

Clocks can be depended on in iOS applications and frameworks starting
with iOS 13+, but the `Clock` protocol is only available in iOS 16+.
Let's add a comment to clarify why (prompted by discussion: #14).

Currently, "unimplemented" clocks are simply immediate clocks that emit
XCTest failures, but the immediacy can be surprising. In general
"unimplemented" versions of dependencies (UUID, Date, etc.) simply call
out to the live dependency, but fail, so we should provide a way to do
the same. Now we can, for example:

```swift
UnimplementedClock(.continuous)
```

An idea suggested
[here](pointfreeco/combine-schedulers#79).
Because we reuse `Task.megaYield` in Clocks, Combine Schedulers, and
TCA, we can use an environment variable to unify them.