Each Interpreters now has a workspace `~/.dyna/tmp/$PID`. This directory is for
any files used by the code generator, REPL, etc.
Many changes

* Support for `=` aggregator. #23

* BUGFIX: when --plan is specified (sorry :-X)

* tweak to error handling. when an item's aggregator fails we set it's value to
  $error instead of it's last "ok" value.

  the repl also show this item as "changed"

* tweaks to doctest runnner

* attempt at contains/2 for list membership -- will eventually change to infix
  operator `in`

* cons and nil can't be assigned values.
trace available at repl supports viewing derivations each item matching query

    :- a :- b.
    :- b:- c.
    :- c.
    =============
    a := true
    b := true
    c := true
    :- trace a

    a = true
    |
    └─ :- true
       # a :- b.
       # a :- b. where {}
       head:   &a
       result: b=true
       |
       └─ b = true
          |
          └─ :- true
             # b:- c.
             # b:- c. where {}
             head:   &b
             result: c=true
             |
             └─ c = true
                |
                └─ |= true
                   # c.
                   # c. where {}
                   head:   &c
                   result: &true