sequence/c changes

- build the sub projections only once and build them before getting
  the blame objects (instead of after getting the actual values)
- added context information to the blame objects
- added an optional #:min-count argument to insist on a minimum length
  for the stream

related to PR 13709
add missing for-label require
IN PROGRESS: working on syntax error messages for 2d
IN PROGRESS: add opt/c to with-contract and provide/contract