restructure fs

std/core/itoa.asm

@@ -0,0 +1,23 @@
+section .bss
+	buffer resb 20
+
+section .text
+	global itoa
+
+itoa:
+	mov rax, rdi
+	mov rsi, buffer + 19
+	mov byte [rsi], 0
+	dec rsi
+.loop:
+	xor rdx, rdx
+	mov rcx, 10
+	div rcx
+	add dl, '0'
+	mov [rsi], dl
+	dec rsi
+	test rax, rax
+	jnz .loop
+	inc rsi
+	mov rax, rsi
+	ret