So that e.g. File.Read could be canceled.

This is already so in fs/ package. However WCFS in Wendelin.core still
uses older nodefs package, and for now it was faster to teach old
nodefs.File about cancel propagation.

References:

wendelin.core@b17aeb8c
wendelin.core@f05271b1
wendelin.core@5ba816da
go123@7ad867a3
go123@0bdac628
go123@d2dc6c09