feat: replace M4 callback writers with safe handles

fec/std/fmt.fe

@@ -1,4 +1,6 @@
 unit fmt;
-pub fn write_str(w: &mut io.Writer, s: str) -> !void;
-pub fn write_int_i32(w: &mut io.Writer, v: i32) -> !void;
-pub fn write_bool(w: &mut io.Writer, v: bool) -> !void;
+pub fn fmt_int_i32(buf: []mut u8, v: i32) -> str;
+pub fn fmt_hex_i32(buf: []mut u8, v: i32) -> str;
+pub fn fmt_char(buf: []mut u8, v: char) -> str;
+pub fn fmt_bool(buf: []mut u8, v: bool) -> str;
+pub fn fmt_error(buf: []mut u8, v: core.Error) -> str;

fec/std/io.fe

@@ -1,9 +1,10 @@
 unit io;
-pub struct Writer {
-    ctx: *void,
-    write_fn: fn(*void, []u8) -> !usize,
-}
+pub enum Writer { Stdout, Stderr, File(u16), Null }
+pub enum Reader { Stdin, File(u16) }
+pub fn write(w: Writer, bytes: []u8) -> !usize;
+pub fn read(r: Reader, bytes: []mut u8) -> !usize;
 pub struct File {
     handle: u16,
-    pub fn close(self: &mut Self) { }
+    pub fn close(self: Self) -> !void;
+    pub fn drop(self: &mut Self) { }
 }