edit: do not share eval context among hook members (#515)
edit: do not share eval context among hook members (#515)

Shared context created an internal dependences between members in the
same hook, especially when the previous member willing to modify it,
e.g. a closure will always reset local scope to its lexical one.
elves/elvish,elves/elvish,elves/elvish,elves/elvish,elves/elvish