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/build.sh

@@ -22,9 +22,6 @@ clang $CFLAGS -o build/evaluator_tests \
 clang $CFLAGS -Itests -c -o build/reader_tests.o tests/reader_tests.c
 clang $CFLAGS -o build/reader_tests \
 	  build/reader_tests.o build/lib.a build/testing.o
-clang $CFLAGS -Itests -c -o build/repl_tests.o tests/repl_tests.c
-clang $CFLAGS -o build/repl_tests \
-	  build/repl_tests.o build/lib.a build/testing.o
 
 # Build application
 clang $CFLAGS -c -o build/main.o app/main.c