Speed up hull

[dpsanders]

No description provided.

[OlivierHnt]

In #664 I added a feature of this PR: the resulting decoration does not depend on decoration(r).

In the current status of this PR, I think this is the only small improvement we can get.
For instance, removing isempty_interval(x) & isempty_interval(y) && return x breaks a few things since:

• internally the bounds of an empty interval are NaN (unless we are using intervals with rational bounds); this was done for extra safety IIRC since NaN propagates
• but publicly, inf and sup on an empty interval return Inf and -Inf respectively; to comply with the standard

Unless you are still trying things out, I suggest to close this PR.

[dpsanders]

What does removing isempty_interval(x) & isempty_interval(y) && return x break?

[dpsanders]

Oh, I see what you're saying :/ That... is unfortunate. Closing for now, thanks.