transition to new type descriptor
stylistic changes

fetch_car => car
fetch_cdr => cdr
make_pair => cons
transition to new type descriptor - remove cruft
stylistic changes

Unspecified => unspecific
Nil         => null_obj
True        => true_obj
False       => false_obj
improve .gitignore
stylistic changes

String => string
restructure data.h & data.c
integrate continuation-capable evaluator

Regressions:
 * NO MACRO-COMPILATION MODE
 * Regressions in error handling and reporting
 * Disabled built-in procedures:
   - Standard procedures dealing with strings, characters and symbols
   - APPLY, MACROEXPAND, EVAL, QUIT
refactor representation of class AstNode
macro-compilation is back
MACROEXPAND-1 is back
APPLY is back
QUIT is back
revise error-reporting
add Reporter to evaluator API
revise primitve procedure API - error-handling returns
remove lava flow
EVAL is back
standard procedures for strings, characters and symbols are back