Use staged functions in @defVar

[joehuchette]

Ref #346, this adds a runtime version of our @gendict macro so that

@defVar(m, x[1:3])

and

T = 1:3
@defVar(m, x[T])

produce the same object (that is, x::Vector{Variable}). This implementation is type unstable, which I think is inevitable, given that we produce different types for T=1:3 and T=2:3. In the benchmarks updates I pushed, this lead to about a 4x performance hit in the worst-case, though there may be more serious performance hits in other settings.

This PR takes the current behavior, which has opaque difference in semantics, with something that has the same behavior, but opaque differences in performance. If the performance hit is just 4x, I think this may be a reasonable change (and could get better without all the evaling once we switch to proper JuMPArrays), but I'm not certain.

[mlubin]

Why don't we just switch to the JuMPArray code now instead of further hacking? Did you want to get this into the 0.10 series?

[joehuchette]

This seems a little too feature-y to put in 0.10.x. I'm fine trying to change JuMPArrays now, but that won't subsume this, just change some of the implementation.