Fix 076, add 077,078 sentinels and many-item pointers

exercises/078_sentinels3.zig

@@ -0,0 +1,27 @@
+//
+// We were able to get a printable string out of a many-item
+// pointer by using a slice to assert a specific length.
+//
+// But can we ever GO BACK to a sentinel-terminated pointer
+// after we've "lost" the sentinel in a coercion?
+//
+// Yes, we can. Zig's @ptrCast() builtin can do this. Check out
+// the signature:
+//
+//     @ptrCast(comptime DestType: type, value: anytype) DestType
+//
+// See if you can use it to solve the same many-item pointer
+// problem, but without needing a length!
+//
+const print = @import("std").debug.print;
+
+pub fn main() void {
+    // Again, we've coerced the sentinel-terminated string to a
+    // many-item pointer, which has no length or sentinel.
+    const data: [*]const u8 = "Weird Data!";
+
+    // Please cast 'data' to 'printable':
+    const printable: [*:0]const u8 = ???;
+
+    print("{s}\n", .{printable});
+}