CORE: Use boost::multiprecision::number API

[afabri]

Summary of Changes

Currently CGAL_Core uses the low level API of GMP for BigInt and BigRat. In this PR I replace this with the boost::multiprecision::number API, so that we can easily switch between the GMP and the cpp backends.

Release Management

• Affected package(s): CGAL_Core
• Issue(s) solved (if any): fix #0000, fix #0000,...
• Feature/Small Feature (if any):
• Link to compiled documentation (obligatory for small feature) wrong link name to be changed
• License and copyright ownership:

TODO:

• document Boost 1.70 is required.
• Announce (CHANGES.md) the boost version bump

[afabri]

There are currently still several assert(false) where I have to rewrite code. And several comments starting with AF:.

[afabri]

I am wondering if we can remove this file with the functions that serialize the various CORE types. This is not documented.

[sloriot]

Since I see "Still under constructions" in the comments, I'd remove it.

[mglisse]

With Sébastien

[sloriot]

Successfully tested in CGAL-6.0-Ic-200

[lrineau]

Is this pull-request (marked Tested ) still a work-in-progress, or can it be merged?

[github-actions]

This pull-request was previously marked with the label Tested, but has been modified with new commits. That label has been removed.

[sloriot]

It can be merged now.