[with patch, with positive review] add a cached_method decorator

[mhansen]

mhansen: Does anyone feel up for reviewing #3781 for me?
[4:21pm] ncalexan: I'll look at it, one moment.  I've wanted 
this for a while.
[4:22pm] mhansen: Awesome.  It doesn't work on C extension 
types though since they don't have a __dict__.  This could 
be done by storing the cache in the decorator object with 
a weakref though.
[4:22pm] ncalexan: The problem is much more complicated 
than this.
[4:23pm] ncalexan: Okay, there are other problems too, 
like un-hashable arguments will break it.
[4:23pm] mhansen: Yep
[4:23pm] ncalexan: And there is no way to clear the cache...
[4:23pm] ncalexan: And the tests don't actually demonstrate 
that the cache is workin.
[4:24pm] ncalexan: (One could touch the cache with an 
incorrect answer, then verify it is "correctly" returning 
that value)
[4:25pm] ncalexan: For what it is, though, it's fine.  It 
will hurt nothing -- shall I review positive?
[4:26pm] mhansen: If you could, that'd be great.  I do 
know it's limitations, but there are some big patches 
going in that depend on it.  I'll make a ticket with 
your comments for improvement.
[4:28pm] ncalexan: How big are the big patches?  In fact, 
I don't care -- this declares the intent nicely and can be 
upgraded independently later.  One moment.

[mabshoff]

Merged in Sage 3.1.alpha1