Convert from StepRangeLen to StepRange

[jishnub]

Currently, one may convert from a StepRangeLen to a StepRange by using the fully parameterized constructor:

julia> r = StepRangeLen(3, 1, 4)
3:1:6

julia> StepRange{Int,Int}(r)
3:1:6

This PR adds a few other constructors that have fewer parameters specified. These parameters may be derived from the argument. After this PR, the following work:

julia> StepRange(r)
3:1:6

julia> StepRange{Int32}(r)
3:1:6

julia> StepRange{Int32}(r) |> typeof
StepRange{Int32, Int64}