new approach to tree rendering
use NodeRenderer as a proper type class
NodeRenderer instances for Pipeline and its components
label Results in the output; show '...' only if there are actually more results that aren't shown
Merge pull request #78 from mossprescott/usability-2

Small improvement to the way results are shown in the REPL
Merge pull request #77 from mossprescott/prettify

new approach to tree rendering
Merge branch 'master' of github.com:slamdata/slamengine into between
closer to a correct ExprOp for Between; no-op for Betwen in Pipeline phase