Use sensible maths for `from_rational` (#13660)

* Use sensible maths for from_rational

* Fixes

* Fixes

* More fixes

* Remove debugging

* Add fuzzer tests

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Prevent panics

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* docs

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Clean up old code

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Test all rounding modes of from_rational

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Clean up code

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Revert "Prevent panics"

This reverts commit 7e88ac76138a1b590e68b68318505b69efe1e1f6.

* fix imports

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* cleanup

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Fuzz test multiply_rational

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Fix import

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* fmt

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Return None in multiply_rational on zero div

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>