Add reactor-cpp as a submodule and reorganize the C++ build process

[cmnrd]

This PR brings the following changes:

1. Make reactor-cpp a submodule of LF.
2. Overhaul the C++ build process (This has become necessary to support 1. while still allowing mixing of multiple reactor-cpp versions)
3. Make C++ code generation smarter by only overwriting files if they actually change. This enables cmake to properly detect that it does not need to rebuild a file and significantly speeds up compilation times after making changes to an LF file.

The new C++ build process

Consider this LF project structure:

src/
    A.lf
    B.lf

Where A.lf and B.lf both contain a main reactor.

Previously, compiling A.lf would create a completely isolated cmake project in src-gen/A/. With this PR, this structure is created instead:

src-gen/
    A/
        CMakeLists.txt
        <source files>
    CMakeLists.txt
    reactor-cpp-lfbuiltin

The internals of src-gen/A/ are similar to before. However, we get a new top-level CMakeLists.txt that creates a top-level cmake project for the entire LF project. A/ is automatically included as a subproject. The directory reactor-cpp-lfbuiltin is automatically copied from the reactor-cpp submodule. The -lfbuiltin suffix indicates that it is the built-in reactor-cpp version of LF.

Let's assume we now compile B.lf and it sets this target property: runtime-version: foo. This will modify the src-gen directory as follows:

src-gen/
    A/
        CMakeLists.txt
        <source files>
    B/
        CMakeLists.txt
        <source files>
    CMakeLists.txt
    reactor-cpp-foo
    reactor-cpp-lfbuiltin

The reactor-cpp-foo directory will be automatically fetched via git. The sub-project B is configured to depend on reactor-cpp-foo, while A still depends on reactor-cpp-lfbuiltin.

Overall, this change makes the build-process a lot more robust. CMake is aware of all the files within a project and knows exatly what needs to be compiled when.

I am not sure if these changes might interfere with the LSP linting. @petervdonovan could you check?

[cmnrd]

While working on this, I noticed that many methods in FileConfig should better be factored out in a FileUtils class, as they only provide file system access without depending on any configuration. I will make this change in a separate PR as to avoid hiding the changes made in this PR.

[petervdonovan]

This sounds like a big improvement! Cloning the Git repository has definitely been a slow point in the language server. From your description it sounds like it probably will break CppValidator since the CppValidator depends on variables in CMakeCache.txt. I will take a look as soon as I can.

[petervdonovan]

With the changes I just pushed, the language server seems to work -- I checked by merging this into the lsp branch locally. LSP tests also pass for me locally, so hopefully there are no more blockers on that front.

[cmnrd]

Thanks for your fixes! I will clean things up then and get it to work in Epoch.

[cmnrd]

As far as I am concerned, this is ready to be merged. The copy mechanism now also works from Epoch and was (manually) tested. It would be great if we could include this PR in the 0.1.0 release. I would only need an approval ;)

[lhstrh]

Look very good! Would the directory name reactor-cpp-default maybe be better than reactor-cpp-lfbuiltin? The latter looks a bit odd and difficult to read, IMO.

[cmnrd]

Yes, probably default is better