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/include/reader.h

@@ -3,6 +3,8 @@
 
 #include "memory_pool.h"
 
+int read_line(int (*get_byte)(void), char *buffer, int buffer_size);
+
 const expression_t *
 read_expression(memory_pool_t *pool, const char *input, int len);
 

lib/include/repl.h

@@ -3,23 +3,11 @@
 
 #include "memory_pool.h"
 
-#define REPL_LINE_BUFFER_SIZE 128
-#define REPL_RESULT_BUFFER_SIZE 128
-
-typedef int (*get_byte_fn_t)(void);
-typedef const expression_t *(*read_fn_t)(
-    memory_pool_t *pool, const char *input, int len);
-typedef int (*evaluate_fn_t)(const expression_t *expression);
-typedef void (*print_fn_t)(const char *output, int len);
+#define REPL_BUFFER_SIZE 128
 
 typedef struct {
-	get_byte_fn_t get_byte;
-	read_fn_t read;
-	evaluate_fn_t evaluate;
-	print_fn_t print;
 	memory_pool_t pool;
-	char line_buffer[REPL_LINE_BUFFER_SIZE];
-	char result_buffer[REPL_RESULT_BUFFER_SIZE];
+	char buffer[REPL_BUFFER_SIZE];
 } repl_t;
 
 bool step_repl(repl_t *repl);