`nit_dir` as a service

ToolContext expose a `nit_dir` service that tools can use.
It is so good that now, the `bin` directory can be directly put in the PATH on most system while `NIT_DIR` is unset.

Previous direct users of `NIT_DIR` are transformed to use the new service.
Also, `nitunit` uses the new service to find the compiler instead of a hard-coded path.

Yet another alternative to #392 and #408