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.