Genericized PackedIntArray and co to PackedArray.

[tgschultz]

PackedArray will accept packed structs too.
Cleaned up documentation.

[andrewrk]

The CI failure is zig fmt related:

+ release/bin/zig fmt --check ..
../lib/std/packed_array.zig

[tgschultz]

Interesting failure: the packed struct test is failing on MIPS, which I suspect is due to endianness behavior. I can't think of a reason that would be though. I'm going to have to set up a way to test MIPS locally to work it out.

[tgschultz]

I believe the compiler's behavior regarding @bitCast on big endian is incorrect. I have described the issue in #9914. I believe it is possible to hack around this, but for the moment I consider this PR blocked.

[tgschultz]

After extensive experimentation, I have determined that big endian packed structs are so utterly broken that I cannot even work around the problem by working with the bytes directly. When creating a struct instance from bytes directly, even when the resulting in-memory representation is identical to the original, the member values are interpreted differently.

For the current version of zig, genericizing PackedArray is hopeless. Therefore I am closing this PR. I will create a new one with the simplifications and documentation improvements but without support for packed structs.