Fix Colon(::TracedRNumber, ::TracedRNumber)

[mofeing]

fixes #837

cc @jumerckx

[Pangoraw]

Note that the test passes without the fix because @trace for does just look for the syntactic variables (start and stop in this case) and does not call :.

[mofeing]

exactly, this fixes the construction of start:end when both vars are TracedRNumbers, which tests weren't checking

[mofeing]

okay, so the solution i propose here doesn't really work because UnitRange{T} requires T<:Real, so UnitRange{TracedRNumber{Int}} is not valid because !(TracedRNumber{Int}<:Real) (too bad Julia is not covariant)