Split some tests out of numbers

[kshyatt]

Moved a bunch of Rational specific tests out into their own new file, split some rounding tests out into that file. numbers.jl is really long and it might be nice to unspool it a bit?

[bramtayl]

In numbers.jl tests like this:

    @test bin(typemin(Int16)) == "-1"*"0"^15
    @test bin(typemax(Int16)) == "1"^15
    @test oct(typemin(Int16)) == "-100000"
    @test oct(typemax(Int16)) == "77777"
    @test dec(typemin(Int16)) == "-32768"
    @test dec(typemax(Int16)) == "32767"
    @test hex(typemin(Int16)) == "-8000"
    @test hex(typemax(Int16)) == "7fff"
    @test string(typemin(Int16)) == "-32768"
    @test string(typemax(Int16)) == "32767"
    @test base(3,typemin(Int16)) == "-1122221122"
    @test base(3,typemax(Int16)) == "1122221121"
    @test base(12,typemin(Int16)) == "-16b68"
    @test base(12,typemax(Int16)) == "16b67"

Are repeated a lot, and could probably be factored out with a representation_profile function which would return a vector of these type mins and maxes. Something I noticed when working on the oct etc. deprecations (a total pain)