This is to separate the three 'call' parameters (target, argv, ctx) from
the 'return to' parameter (k). The old order made it look as if the dynamic
context was in some way related to the continuation, which is not the case.
Fix tree_replace() to handle recursive data structures.
Fix some other minor bugs in the reader and interpreter.
Implement comment-escapes in the string parser, for more readable input.
Allow input program files (*.rla) to be invoked directly, with arguments.
Add a simple string->number converter as a builtin function.
Jesse D. McDonald