Speed up a..b

[dpsanders]

Don't use complicated conversions -- leave that for @interval.
Just do Interval(prevfloat(a), nextfloat(b)).

But can treat integers exactly:

..(a::Integer, b::Integer) = Interval(a, b)
..(a::Integer, b::Float64) = Interval(a, nextfloat(b))
..(a::Float64, b::Int) = Interval(prevfloat(a), b)

..(a::Float64, b::Float64) = Interval(prevfloat(a), nextfloat(b))

This will take care of most of the useful cases, such as writing 0..\infty.

[dpsanders]

This gives

julia> @format full
6

julia> 0..1
Interval(0.0, 1.0)

julia> 0.0..1.0
Interval(-5.0e-324, 1.0000000000000002)

julia> 0..∞
Interval(0.0, Inf)

[dpsanders]

See PR #37