Doc B
Implem XOR via /=

Closes #5
make check_clean: don't fail if already clean
Refactor logic ops
Refactor arith ops
don't try to compile old code
Tests for comparisons & boolean ops
ignore compiled tests
Tests unops
dedup print_bid, print_idlist_comma, print_value
dedup print_op_*
extra print_idlist_comma
extra print_value
with_env
dedup print_expr
dedup print_basetype
one less "assert false"
coverage: add printer
print_elt_list = print_idlist_comma
dedup print_*_list