Copyrights header and licensing

[ljmf00]

To cover both copyrights from The D Language Foundation and Taurus library I propose to use the same open-source license of Phobos, DMD and DRuntime, Boost 1.0 (BSL-1.0).

For consistent file licensing, we could adopt the SPDX standard and have an SPDX-License-Identifier in every relevant source code file in the first line, following SPDX specification v2.2.0, Appendix V.

An example of the header file:

// SPDX-License-Identifier: BSL-1.0

/*
 _____                          
|_   _|                         
  | | __ _ _   _ _ __ _   _ ___ 
  | |/ _` | | | | '__| | | / __|
  | | (_| | |_| | |  | |_| \__ \
  \_/\__,_|\__,_|_|   \__,_|___/

Copyright (C) 2021 Taurus library.

 */

/**
 * Module documentation
 */

...

[ljmf00]

Maybe we can add a brief about the license, although, with SPDX I don't see it necessary.

[iK4tsu]

We should add disclaimer about Phobos, DRuntime and the D Language Foundation, with basic information, acknowledgement of some feature implementations and copyright.