ENH: Optimise operators which preserve the input

[pchanial]

Operators such as ReshapeOperator or SliceOperator preserve their inputs (let's call such an operator PRE).
Currently, when composed by an out-of-place operator OUT:

y = (OUT * PRE)(x)

the following operations take place:

tmp = PRE(x)
y = OUT(tmp)

The tmp variable should be optimised out. I suggest a 'preserve_input' decorator to flag these operators.