all: migrate code to new cast builtin syntax

Most of this migration was performed automatically with `zig fmt`. There
were a few exceptions which I had to manually fix:

* `@alignCast` and `@addrSpaceCast` cannot be automatically rewritten
* `@truncate`'s fixup is incorrect for vectors
* Test cases are not formatted, and their error locations change

test/cases/safety/pointer casting to null function pointer.zig

@@ -13,7 +13,7 @@ fn getNullPtr() ?*const anyopaque {
 }
 pub fn main() !void {
     const null_ptr: ?*const anyopaque = getNullPtr();
-    const required_ptr: *align(1) const fn () void = @ptrCast(*align(1) const fn () void, null_ptr);
+    const required_ptr: *align(1) const fn () void = @ptrCast(null_ptr);
     _ = required_ptr;
     return error.TestFailed;
 }