add documentation for packed structs (bitfields)

[andrewrk]

This issue extracted from #367

• packed structs, packed enums, packed unions
• ability to @bitCast at comptime and @ptrCast at comptime
• mention that normal structs do not guarantee what size they are, the field order, or padding.
• zig supports arbitrary integer types
• normally integer types smaller than 8 bits would still be 1 byte, but in packed structs they are packed
• bools count as 1 bit
• taking a pointer to a packed struct field has special syntax
• mention that the address of a pointer to a packed struct field can be the same address for different fields
• rename @offsetOf to @byteOffsetOf and add @bitOffsetOf #633 @byteOffsetOf and @bitOffsetOf
• talk about alignment. packed structs have 1 byte alignment. but if you have a pointer to a packed struct that is overaligned, zig should still understand alignment of fields when appropriate. related: ability to set alignment on struct fields #1512
• mention that a load/store - especially volatile - might codegen to extra code than if you do it manually. because zig will perform a volatile store 2x, whereas manually you would do the bit shifting once and then write the entire byte at once.