Update of $OutputSizeLimit

[poke1024]

This is a cleaned up, faster and better version of $OutputSizeLimit.

It now lives in its own class based on _MakeBoxesStrategy and makes the state more clean by using a dedicated class _LimitedMakeBoxesState. The whole code is now in expression.py to avoid the ugly in procedure imports needed for Expression until now.

There are two functional differences from the current code:

(1) The very first and the very last elements in a tree are now always output; e.g. Nest[Partition[#, 2]&, Range[2 ^7], 7] now yields {{{{{{{{1, <<1>>}, <<1>>}, <<1>>}, <<1>>}, <<1>>}, <<1>>}, {<<1>>, {<<1>>, {<<1>>, {<<1>>, {<<1>>, {<<1>>, 128}}}}}}}} (it yields a lame {<<1>>} with the current implementation).
(2) Every output that has been calculated using MakeBoxes is now always output and never thrown away.

(2) is a relaxation from the strict limit given by $OutputSizeLimit. The effect of this is a big speedup, since it no longer retries different parts of the expression to fit the remaining capacity, but simply outputs one and then stops. Try setting x=Nest[Partition[#, 2]&, Range[2 ^10], 10] and then outputting x several times; the current code is very sluggish (> 2s on my machine), this new code is fast (<.5 s).