Refined Context consume strategy and add UserData and is_consumed to Error structure

[ehwan]

Context is now consumed, so the context can't be reused after consumed for some feed failure:

• for NoAction, since the stack was not modified, the Context still can be reused after failure
• for ReduceActionError, since the stack was modified, the Context can't be reused after the error.
• If the user tried to feed on consumed Context, it will immediately return the error, and can_feed and it's variant will always return false on consumed Context.
• accept() now takes &mut self instead of self. When accept succeeded, the context will be switched into consumed state.