restructure fs

std/core/arr_size.asm

@@ -0,0 +1,6 @@
+global arr_size
+
+section .text
+arr_size:
+	mov rax, [rdi]
+	ret

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

std/core/str_len.asm

@@ -0,0 +1,13 @@
+global str_len
+
+section .text
+str_len:
+	xor rax, rax
+.loop:
+	cmp byte [rdi], 0
+	jz .done
+	inc rax
+	inc rdi
+	jmp .loop
+.done:
+	ret