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.

app/main.c

@@ -1,21 +1,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#include "evaluator.h"
-#include "reader.h"
 #include "repl.h"
 
-static void print(const char *output, int len)
-{
-	fwrite(output, 1, len, stdout);
-}
-
-static repl_t repl = {
-	.get_byte = getchar,
-	.read = read_expression,
-	.evaluate = evaluate,
-	.print = print,
-};
+static repl_t repl;
 
 int main(void)
 {