Currently, packed structs are always align(1) and each field of a packed struct is either align(1) or a special kind of alignment that indicates bit offsets. I propose the following changes:

• implement ability to specify explicit field alignment of packed structs (related to ability to set alignment on struct fields #1512), including even bit offsets. This can cause padding to be inserted (possibly even just bits to the next byte).
• packed structs, like normal structs, have the alignment of the most-aligned field. Minimum 1 byte.
• without explicit alignment specified, fields get bit packed. However fields can be specified to be byte aligned and pointers to these fields can even be naturally aligned. A packed struct with every field like this: field: T align(T), would be equivalent to an extern struct.
• ability for packed structs to have fields that are normal structs. In this case whether a packed struct has well-defined memory layout is determined by whether or not all the fields it has have well-defined memory layout. Normal structs and types can still be bit-packed though!
• when a packed struct has no well-defined memory layout, it can participate in debug safety features (see add safety checks for pointer casting #2414). Perhaps there could even be a special type that could be used as a field, that allows Zig to put the debug safety in that field's location, and packed structs would have well-defined memory layout until the first field that does not have well defined memory layout. This would allow putting this debug safety field at the end, and still doing @bitCast, @ptrCast, or other useful things with packed structs.
• fix the issues regarding packed structs not ending with byte-alignment. This should be handled semantically equivalent to, for example, u7.

Related bug: #2627