feat: align M3 slices and strings with v0.1.7

fec/tests/m3/bad-mlet.fe

@@ -0,0 +1,6 @@
+unit m3_bad_mut_let;
+
+fn bad() -> void {
+    var raw: [2]u8 = [1, 2];
+    let s: []mut u8 = raw[..];
+}

fec/tests/m3/bad-shwr.fe

@@ -0,0 +1,5 @@
+unit m3_bad_shared_write;
+
+fn bad(s: []u8) -> void {
+    s[0] = 1;
+}

fec/tests/m3/mutable.fe

@@ -0,0 +1,11 @@
+unit m3_mutable;
+
+fn takes_shared(s: []u8) -> u8 { return s[0]; }
+
+fn main() -> i32 {
+    var raw: [3]u8 = [1, 2, 3];
+    var s: []mut u8 = raw[..];
+    s[1] = 9;
+    if takes_shared(s) == 1 and raw[1] == 9 { return 0; }
+    return 1;
+}

fec/tests/m3/slcbound.fe

@@ -0,0 +1,7 @@
+unit m3_slice_bounds;
+
+fn main() -> i32 {
+    let a: [2]i32 = [1, 2];
+    let s: []i32 = a[0..3];
+    return s.n as i32;
+}