WIP env test: 'rewriting is ok'
Move eval into try block

This way the interpreter won't dump core when an exception is raised
during eval. Later this shouldn't be necessary when I get rid of
exceptions.
Refactoring read_pair, partly done
Fix broken list test: (0 ())

It parses a list of length two, not a pair of 0 and empty:
(cons 0 (cons empty (cons empty)))
Finish refactoring read_pair for
unsure about quoting lists
forgot to remove some commented code
How to fix defines, and more upcoming tests

Also print the heap after the interpreter is done running, for
inspection purposes.
cool compiler, linker flags for Makefile
start unit testing the lists code
define can rewrite

std::map::insert will not overwrite, had to check for this condition and
erase before inserting.
set! success
set! failure
WIP if