Relicensing to MIT license

In agreement with Tohoku University on March 25, 2021, we (the SML# development team) have decided to relicense the SML# software.

From now on (the current revision of the bleeding edge), the SML# software will be distributed under the MIT license copyrighted by the SML# development team. See LICENSE file for the current license. Note that the past releases (3.7.1 and prior) are not relicensed; the same as before, they are distributed under 3-clause BSD license copyrighted by Tohoku University.

We believe that this change makes it easier for people other than Tohoku University to join the SML# project as an open-source software project. We also believe that the MIT license, which does not contain the binary redistribution clause, allows the users to use SML# more freely.