Preserve SECD registers across _dumpstate
Add DROP function to Util.lso
Add stream processing functions

Functions to handle CONS cells with potentially DELAYed components.
Refactor scheme lexer

Scheme lexer no longer requires a continuation to be passed in.
Instead, it returns a list with a DELAYed CDR, and forcing the CDR
implicitly causes the lexer to be re-entered.
Add Landin's J-operator

For encapsulating the state of the SECD machine as a closure for later
invocation -- used to implement Scheme's "call/cc" operator.

See:

P.J. Landin. A generalization of jumps and labels. Report, UNIVAC
Systems Programming Research, August 1965.

H. Thielecke. An introduction to Landin's "A generalization of jumps
and labels", Higher-Order and Symbolic Computation 11(2):117-123, 1998
Implementation of Scheme's "call/cc" operator
Yin-yang puzzle (example using CALL/CC)
Reimplement PARSE_OR using CALL/CC

This guarantees that if the first alternative (P) succeeds, that Q will
not be invoked.