CORE: Accelerate writing to a stream if precision <= 17

[afabri]

Summary of Changes

Writing an Exprto as stream is slow. We make it faster by writing Expr::doubleValue() if the precision is smaller or equal to 17.

Release Management

• Affected package(s): CORE
• License and copyright ownership: unchanged

[sloriot]

@afabri CGAL-6.0-Ic-238 includes your latest commit. You can have a look at the results and remove the last commit.

[afabri]

@sloriot please retest so that we can merge

[sloriot]

Successfully tested in CGAL-6.0-Ic-245