feat: replace M4 callback writers with safe handles

fec/tests/m4/bad-bufw.fe

@@ -0,0 +1,7 @@
+unit m4_bad_buffer_writer;
+
+fn main() -> void {
+    var raw: [4]u8 = [0, 0, 0, 0];
+    var buf: []mut u8 = raw[..];
+    let w = io.buf_writer(buf);
+}

fec/tests/m4/bad-writ.fe

@@ -2,6 +2,6 @@ unit m4_bad_writer;
 
 fn main() -> i32 {
     var x: i32 = 0;
-    @fprint(&mut x, "bad");
+    @fprint(x, "bad");
     return 0;
 }

fec/tests/m4/format.fe

@@ -8,15 +8,15 @@ fn main() -> i32 {
     var raw3: [8]u8 = [0, 0, 0, 0, 0, 0, 0, 0];
     var raw4: [8]u8 = [0, 0, 0, 0, 0, 0, 0, 0];
     var raw5: [8]u8 = [0, 0, 0, 0, 0, 0, 0, 0];
-    var buf: []u8 = raw[..];
-    var buf2: []u8 = raw2[..];
-    var buf3: []u8 = raw3[..];
-    var buf4: []u8 = raw4[..];
-    var buf5: []u8 = raw5[..];
-    let w: io.Writer = io.buf_writer(&mut buf);
+    var buf: []mut u8 = raw[..];
+    var buf2: []mut u8 = raw2[..];
+    var buf3: []mut u8 = raw3[..];
+    var buf4: []mut u8 = raw4[..];
+    var buf5: []mut u8 = raw5[..];
+    let w: io.Writer = io.null_writer();
     const LOCAL_FMT: str = "value={}\n";
     @print(FMT, 7, 15, 'A', "yes", true);
-    @fprint(&mut w, LOCAL_FMT, 12);
+    @fprint(w, LOCAL_FMT, 12);
     let n: usize = @sprint(buf2, "A\x42\u0043defghi");
     let n2: usize = @sprint(buf3, "xy");
     let inner_n: usize = @sprint(buf4, "xy");

fec/tests/m4/prop.fe

@@ -1,5 +1,5 @@
 unit m4_prop;
 
-pub fn propagate(w: &mut io.Writer) -> !void {
+pub fn propagate(w: io.Writer) -> !void {
     try @fprint(w, "a{}b", 1);
 }

fec/tests/m4/proptest.c

@@ -1,23 +1,11 @@
 #include "prop.c"
 
-static unsigned short calls;
-
-static unsigned short fail_write(void *ctx, const unsigned char *p,
-                                  unsigned long n)
-{
-    (void)ctx;
-    (void)p;
-    (void)n;
-    ++calls;
-    return calls == 1 ? 7 : 0;
-}
-
 int main(void)
 {
     fe_writer w;
     unsigned short result;
-    w.ctx=0;
-    w.write_fn=fail_write;
-    result=fe_m4_prop_propagate(&w);
-    return (result==7 && calls==1) ? 0 : 1;
+    w.tag=2;
+    w.handle=99;
+    result=fe_m4_prop_propagate(w);
+    return result==1 ? 0 : 1;
 }

fec/tests/m4/try-fpr.fe

@@ -2,7 +2,7 @@ unit m4_try_fprint;
 
 fn main() -> !void {
     var raw: [4]u8 = [0, 0, 0, 0];
-    var buf: []u8 = raw[..];
-    let w: io.Writer = io.buf_writer(&mut buf);
-    try @fprint(&mut w, "ok");
+    var buf: []mut u8 = raw[..];
+    let w: io.Writer = io.null_writer();
+    try @fprint(w, "ok");
 }