Extract line reading logic to reader and remove REPL tests

Reading a line into the buffer was the only logic complex enough to be
worth testing really, and exposing the necessary parts of it to test
it effectively was a pain.  Makes more sense to move read_line out and
throw away most of the tests.

scripts/test.sh

@@ -4,6 +4,5 @@ fails=0
 
 build/evaluator_tests || fails=`expr $fails + 1`
 build/reader_tests || fails=`expr $fails + 1`
-build/repl_tests || fails=`expr $fails + 1`
 
 if [ $fails -eq 0 ]; then echo Tests OK; fi