hdl.ast: warn on unused property statements (Assert, Assume, etc).

hdl.ast: warn on unused property statements (Assert, Assume, etc).

A property statement that is created but not added to a module is
virtually always a serious bug, since it can make formal verification
pass when it should not. Therefore, add a warning to it, similar to
UnusedElaboratable.

Doing this to all statements is possible, but many temporary ones are
created internally by nMigen, and the extensive changes required to
remove false positives are likely not worth the true positives.
We can revisit this in the future.

Fixes #303.