implement basic string functions (memset, memcpy, etc)

src/screen.c

@@ -1,5 +1,6 @@
 // screen.c
 #include "../include/screen.h"
+#include "../include/string.h"
 
 static size_t terminal_row;
 static size_t terminal_column;
@@ -16,14 +17,6 @@ inline uint16_t vga_entry(unsigned char uc, uint8_t color)
         return (uint16_t) uc | (uint16_t) color << 8;
 }
 
-size_t strlen(const char* str) 
-{
-        size_t len = 0;
-        while (str[len])
-                len++;
-        return len;
-}
-
 void terminal_initialize(void) 
 {
         terminal_row = 0;