TODO update

  * read: does not recognize missong close paren on EOF

  * start/end args for dolist/doseq?

  * idea: have a stack for bound symbols, saved values, and number

    of saved bindings each, to be accessed by something like a

    (local-bindings n) function, with n being the number of levels

    to look at. And lo!, together with a stack of the bind contexts

    (lambda, let, etc.) this could give us a *nice* stack trace.

    Investigate.

  @ fix regtests; 024

  @ start/end args for dolist/doseq?

  @ fix regtests; 025

  + better argument checking (number, types) for Builtins (instead

    of is_def() et al.)