(5 Aug 2020, @ankostis): rename return-dict outs; step number badges

FEAT(op): :func:.keyword modifier can rename outputs of a :term:returns dictionary
function.

• fix: rescheduled function err-msg were listing wrong missing provides.

• enh: err-msg did not list returns-dict mismatches.

• ENH(plot): add number badges on operations & data nodes to denote execution order;
  theme show_steps=False can hide them;

  • feat: data-nodes are Graphviz HTML-ized to support badges.

• break(fnop): do not accept operations without any provides - they were pruned before,
  but it is better to fail asap.

  • fix/break: clip extra data from a :term:returns dictionary function - previously,
    full-eviction assertion was kicking.

• enh(plan): why operations are pruned is now explained in the plan & the plot tooltips.

• fix(plan): :meth:.ExecutionPlan.validate() may be called with no args,
  and uses the compiled ones.

  • enh: suggest mode of action on the error message, when graph has cycles;
    top-sort nodes only once, and report them to jetsam (see below).
  • doc: enhance existing tutorial section to explain compilation.

• feat(:term:jetsam): pipeline was not collected, till now.

  • FEAT: items can be accessed as jetsam attributes (helpful for interactive REPLs).
  • ENH: don't "catch" exception, use try-finally with ok flag instead,
    not to corrupt the landing position of a post-mortem debugger.
  • feat: now collecting also pruned_dag, op_comments, sorted_nodes while
    :term:compiling <compile>.
  • revert: stop logging every jetsam item on each salvaging point, not to flood
    logs (which had been introduced in the previous release).
  • refact: move into own module.

• fix(SphinxExt): catch top-level errors that if occurred, message and stack trace
  were lost.

• doc: list anti-features (when to avoid using this lib).