Non-thin interval is displayed as thin interval

[dpsanders]

julia> x = interval(0.1)
[0.1, 0.1]

julia> sin(x)
[0.0998334, 0.0998334]

julia> diam(sin(x))
1.3877787807814457e-17

The interval sin(x) is not thin (the diameter is > 0), but the display shows the same lower and upper bounds.

[dpsanders]

This display code used to use a BigFloat implementation to do directed rounding; that code seems to have been replaced when moving to v1.0. However the result is sometimes wrong :)

I'll file a PR where I suggest to bring back the old approach.

[dpsanders]

Fix in #752 — restores directed rounding (RoundDown for lower bound, RoundUp for upper bound) that was accidentally removed in the 1.0 release.

[dpsanders]

It seems that the bug was that _round_string always rounded to nearest, so when lo and hi of a
non-thin interval were close enough, they would display identically, e.g.
[0.0998334, 0.0998334].

[dpsanders]

We want the displayed interval to always be a valid enclosure.