reworking tags and transformations, stage 1:
- introduce a new Theory.tdecl "Meta" to be used for tags - simplify cloning procedure, get rid of the th_clone field - when a goal proposition is discarded during cloning, it's still keeped in the theory as a "skip proposition", this is needed to preserve/clone every local identifier. Skip propositions are eliminated during task formation. - get rid of a separate Task.tdecl type - reorganize the Task.task_hd record: * use/clone history is cached in a theory-keyed map; * meta-properties are cached in a tagname-keyed map. This is done to simplify the fine-grained configuration of transformations.