Remove PackedIntArray from std

[tgschultz]

As the original author of the packed_int_array module, it is my opinion that it should be removed from std. My reasoning is as follows:

1. I originally wrote it to prove that we didn't need packed int arrays as a language feature during a time when Zig's semantics were quite a bit different
2. (read|write)PackedInt* are a smaller, less cumbersome implementation of much of the solution with the rest being trivially implementable from them
3. Packed structs with @Vectors are also effectively packed int arrays
4. There are several open issues regarding the module (design flaw: semantics of loading values with undefined bits #19634, std.PackedIntArray fails for some int types at comptime #19627, Assertion failed at /deps/zig/src/stage1/codegen.cpp:7611 in gen_const_val #7648)
5. Of the uses of the module that I have seen, none cannot be easily replaced with the above alternatives