Make dependency on 'stylus' optional

[Botond Ballo]

Since the default frontend (and the only one that's currently hooked up) is the command-line one, which doesn't require stylus, stylus should not be a required dependency of the project.

[Botond Ballo]

To be clear, by that I mean that "make all" should complete successfully on a system without stylus installed.

[Botond Ballo]

Reorganize directory structure and makefile. The web frontend is no longer built by default (fixes #15).

→ <<cset 4966da2215c0>>

[Botond Ballo]

As part of this change, I reorganized the directory structure and makefile a bit:

• Created a frontend directory with cmd and web subdirectories for the command-line and web frontends respectively. The cargo project for the command-line frontend now lives in frontend/cmd.
• There are makefile targets cmd-frontend and web-frontend for building the respective frontends.
• make all only builds cmd-frontend (thereby fixing this bug).
• I also added a make check which groups make lint and make test. Conceptually, make check is something developers should run before committing their changes, but something users who just want to build and run the tool should not need to run (nor install the additional dependencies it has compared to make all, like nightly rust).