Deprecate [[lib]] in favor of [lib]

The syntax was originally chosen to perhaps allow multiple libraries in the future, but that is less and less likely to happen at this point. This commit deprecates the now-misleading `[[lib]]` in favor of `[lib]` to indicate that only one can be present.

This does not start allowing `[bin]` or `[example]` and such as I felt that it was too many ways to specify what's essentially the same thing. This also as a bonus allows a top-level `bin = []` to completely opt-out of binary inference (as well as for tests and examples).