Refactored the C files out into libraries, started work on the BASIC repl (it can sorta do math!)

src/string.h

@@ -0,0 +1,12 @@
+#ifndef _STRING_H_
+#define _STRING_H_
+
+#include "types.h"
+
+size_t strlen(char *ptr);
+int strncat(char *dest, char *src, size_t n);
+void *memset(void *s, int c, size_t n);
+void *memcpy(void *dest, void *src, size_t n);
+int strcmp(const char *s1, const char *s2);
+
+#endif /* _STRING_H_ */