When doing `Setup(...)` we automatically setup a `SetupScope` which
turns on the `RecursiveMockBehavior`'s automatic setup of null-valued
returns just as you would have done if you had done
`mock.RecursiveProp.Returns(Mock.Of<IRecursiveProp>)` followed by
the actual setup of `mock.RecursiveProp.Name.Returns("foo")`.

This can now be shrinked to `mock.Setup(x => x.RecursiveProp.Name).Returns("foo")`.

Simplifying the detection of an active setup was simplified since the previous
approach of making it per-mock (we were keeping a state flag in the mock itself)
wouldn't work well in recursive mocks since you'd need to preserve that flag across
mocks, and it wasn't really obvious how much benefit we were getting from making
it per-mock as opposed to something like:

```c#
using (Setup())
{
  // Set up any number of mocks in a single block
}
```

So we enabled precisely that, by moving to a global per-execution context flag
kept by an `AsyncLocal<T>` than can propagate threads.

NOTE: we don't keep track of how many scopes have been created so far and
which ones have been disposed to run off the `SetupScope.IsActive` flag.
Doing so might be useful eventually, but it's better to keep the design simple
for now since having multiple nested calls that would perform setups is
likely quite a rare scenario anyway.