Add #[packed] attribute to create packed structs

[huonw]

#5758 take 2.

This adds a #[packed] attribute for structs, like GCC's __attribute__((packed)), e.g.

#[packed]
struct Size5 {
   a: u8,
   b: u32
}

It works on normal and tuple structs, but is (silently) ignored on enums.

Closes #1704.

[huonw]

@nikomatsakis, how does this look? Unsurprisingly, using the existing infrastructure made the whole thing seem nicer.

I didn't bother with the cache, since it is only used in type_of and adt::represent_type, but both of these already have a cache.

[nikomatsakis]

@dbaupp looks great.