Refactor the project architecture and add tests

[dfintha]

The project currently looks like it evolved organically. A simple, robust architecture should be implemented, functionalities separated into layers, and tests added to each layer, and the in between.

This should be done before #36, the multiplatform support can benefit from these refactors.

[dfintha]

Calls to command-line utilities (git, make, pkg-config, find, ...) could be extracted into a separate module with its own unit tests, to check for presence of the GNU and/or LLVM utilities, and their relevant usage.

[dfintha]

The result module is OK by itself, its just a specialization of std::result::Result and conversion functions for it.

[dfintha]

The output module should be merged into the utilities module.

[dfintha]

In the makefile module, composition of various build steps/flags/dependencies should be extracted into separate functions, and their output should be tested with unit tests.

[dfintha]

Simiar to the makefile module, composition of commands in the project module should be extracted into separate functions, and tested.

[dfintha]

Also refactor-related: the clap crate could be used in a more efficient way than manually building its scheme. Its parsing functionality should also be tested.

[dfintha]

Helper functions could be moved into their relevant structures, and correlating functionalities could be composed into ones.

• BuildScript could be a structure, which can be used in place of ScriptEnvironment and execute_script.
• get_toolset_executables and collect_source_files could be internalized into the Project structure.