[Wassasin]

On the current main branch we state:

Based on a blog by Jyn and completely forked parts of rustc and ferrocene.

Though faithful, this is however not a very precise attribution.

This PR attempts to correct this situation as follows:

1. Add explicit notices to the top of the files which are sourced & adapted from other projects. These include:
   • cga_analysis/src/monomorphize.rs
   • tests/compile-test.rs
   • cga_analysis/src/callgraph/bigraph/bfs.rs (moved into separate file)
2. Add LICENSES/Apache-2.0.txt and LICENSES/MIT.txt, clarify their application in COPYRIGHT. Note that CGA will only remain MIT, but that Apache also applies to the above files. (I would not mind Apache for CGA too in any case)
3. Add FSF REUSE.toml similarly to how rustc does it. The use of this tool is checked in CI.
4. Code that originates from Ferrocene is removed. Basically what the Ferrocene code supplied was the MIR visitor pattern. Very similar code also is used in rustc and clippy. Formally this project is still derived from Ferrocene, even though the code has been removed. This is reflected in REUSE.toml. During the removal process the code has been refactored.

It is unclear to me whether CGA should be licensed under Apache too, and whether the License file should now also include Copyright (c) The Rust Project Contributors, as it is in Ferrocene.