Update libcoro to 1.67
Update stack limits

By default Fibers used to have a stack limit of 64k with 2k padding for
v8's native code. This was problematic because some v8 code can use at
least 240k of stack not even including the stack space client JS uses.
The new limits are 512k/1M per fiber with 128k/256k padding.

Additionally this implements libcoro's new stack management interface.
Fix up Windows support

libcoro now has a Windows Fiber backend, so we can remove the custom
code that was handling that.