toUint256() function should return the number without the floating part

[FabienCoutant]

Thanks to the great lib.

I just have one remark.
I expected the toUint256() function to return the number rounded as Uint256.
For example: passing 12,3456 I expect to get 12 from toUint256()
To get the right result I have to pass my 12,3456 into the toFelt() function and then the toUint256().

Also we can add what type of rounding we want as param to the function.

Best,