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/compile_errors/integer_cast_truncates_bits.zig

@@ -1,6 +1,6 @@
 export fn entry1() void {
     const spartan_count: u16 = 300;
-    const byte = @intCast(u8, spartan_count);
+    const byte: u8 = @intCast(spartan_count);
     _ = byte;
 }
 export fn entry2() void {