I don't expect this to be merged soon, I had to update it because I myself use zigimg and already updated to 0.16.x

Also updates all the tests and they all pass.

Some notable things to note:

• The self-hosted backend crashes when bitcasting packed structs which have 0-bit fields in them (that's why the tests are built with llvm)
• Now pointers to any field in a packed struct are bitpointers, even if byte-aligned (affects ToMethods)
• std.testing.refAllDeclsRecursive has been deleted
• Almost all math functions have to use inline loops as indexing vectors at runtime is now not allowed.

Edit:

• After the type resolution changes looks like zigimg triggers either IB or a miscompilation of tagged unions and as such some tests crash.

Edit2:

• It seems the crashes are gone but some tests still fail due to some miscompilation (very likely) as somehow we end-up in the BMP code-path when writing a TGA.