[RFC] Support object types in BCMath (#13741)

Added BcMath\Number class. It is an immutable object, has methods that are
equivalent to existing BCMath calculation functions, and can also

[RFC] Support object types in BCMath (#13741)

Added BcMath\Number class. It is an immutable object, has methods that are
equivalent to existing BCMath calculation functions, and can also be calculated
using operators.

The existing BCMath function returned a string for each calculation, but this
class returns an object.

RFC: https://wiki.php.net/rfc/support_object_type_in_bcmath,
https://wiki.php.net/rfc/fix_up_bcmath_number_class

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Co-authored-by: Niels Dossche <7771979+nielsdos@users.noreply.github.com>

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