WIP: Added StepRangeLen

[mauro3]

Fixes #374

[timholy]

Maybe...the problem is that StepRangeLen fixes a whole host of issues with FloatRange. It might be better to copy JuliaLang/julia#18777 into Compat...

[mauro3]

Briefly scanning that PR suggests that it would be about 500 LOC to be added to Compat. Is that not excessive? Alternatively, I could add a warning to the README.md that there might be subtle differences?

[mauro3]

Migrating away from FloatRanges in my package, I also had to patch some more bits:

if VERSION < v"0.6.0-dev.2376"
    const StepRangeLen = FloatRange
    @compat const SRangeL{F} = StepRangeLen{F}
else
    @compat const SRangeL{F} = StepRangeLen{F,Base.TwicePrecision{F},Base.TwicePrecision{F}}
end

# this is then ok:
type A{F}
    r::SRangeL{F} # replacing r::FloatRange{F}
end