Make bypassing capture context-aware

[alexhall]

This change makes the capture/bypass macro aware of the capture context in which it's running, so that it can more intelligently restore previous out/err writers (or just pass through if no capturing is active).

Unsupported corner cases:

• Nested capturing (will write to the next level up, not the original out/err)
• Code running on other threads where bindings weren't conveyed (will still write to the redef'ed out/err)

If this approach looks sensible, I'm happy to clean up the code, add tests, etc.

Fixes #446