[stm] Make toplevels standalone executables.

We turn coqtop "plugins" into standalone executables, which will be
installed in `COQBIN` and located using the standard `PATH`
mechanism. Using dynamic linking for `coqtop` customization didn't
make a lot of sense, given that only one of such "plugins" could be
loaded at a time. This cleans up some code and solves two problems:

- `coqtop` needing to locate plugins,
- dependency issues as plugins in `stm` depended on files in `toplevel`.

In order to implement this, we do some minor cleanup of the toplevel
API, making it functional, and implement uniform build rules. In
particular:

- `stm` and `toplevel` have become library-only directories,
- a new directory, `topbin`, contains the new executables,
- 4 new binaries have been introduced, for coqide and the stm.
- we provide a common and cleaned up way to locate toplevels.

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