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.

lib/reader.c

@@ -2,6 +2,7 @@
 
 #include <ctype.h>
 #include <stddef.h>
+#include <stdio.h>
 
 static int parse_expression(
     memory_pool_t *pool, const char *input, int len, expression_t **out);
@@ -152,6 +153,19 @@ static int parse_expression(
 	return used;
 }
 
+int read_line(int (*get_byte)(void), char *buffer, int buffer_size)
+{
+	for (int len = 0; len < buffer_size; ++len) {
+		const int byte = get_byte();
+		if (EOF == byte)
+			return -1;
+		if ('\n' == byte)
+			return len;
+		buffer[len] = (char)byte;
+	}
+	return buffer_size;
+}
+
 const expression_t *
 read_expression(memory_pool_t *pool, const char *input, int len)
 {