Clean up license text
Clean up license text

Small tweaks to license text layout and one change OWNER -> HOLDER
as in https://choosealicense.com/licenses/bsd-3-clause/.

Added "BSD 3-Clause License" headline.

Changed Copyright line to "Copyright 2020 uint256 Authors"
as in https://opensource.org/licenses/BSD-3-Clause.
The "All rights reserved" omitted as it has no use nowadays.

holiman/uint256,holiman/uint256