Added from_bits and into_bits functions.

[Frostie314159]

This PR modifies the bitfield attribute macro to automatically emit from_bits and into_bits functions for types on which it is applied. This makes nesting types a lot easier.

[Frostie314159]

Just after creating this PR I noticed, that this doesn't work for nested types with different sizes, so basically all of them. This could be resolved, by converting the representation of the parent to the next highest power of two, from the bit length of the Child.

[wrenger]

Just after creating this PR I noticed, that this doesn't work for nested types with different sizes, so basically all of them. This could be resolved, by converting the representation of the parent to the next highest power of two, from the bit length of the Child.

Well, yes, this problem was pretty much the reason for not generating them automatically.

What do you mean by your second sentence?
A solution might be to use the member size (rounded up to the next int) instead of the size of the entire bitfield for (from|into)_bits. Or maybe this can be made more generic so that any type smaller than the bitfield size can be used for (from|into)_bits.

(Related to #33)

[Frostie314159]

What I meant was, that since all bitfields are currently backed by integer types it's safe to assume that the type backing a bitfield is the nearest power of two to the bit count specified for the field it was used in.

[wrenger]

So I allowed the conversion functions (from/into) to return different ints from the underlying bitfield type. Now, nested bitfields like the following should be possible (with your PR).

#[bitfield(u8)]
#[derive(PartialEq)]
struct Child {
    contents: u8,
}
#[bitfield(u16)]
#[derive(PartialEq)]
struct Parent {
    #[bits(8)]
    child: Child,
    #[bits(8)]
    __: (),
}
let child = Child::new().with_contents(0xff);
let parent = Parent::new().with_child(child);
assert_eq!(child.into_bits(), 0xff);
assert_eq!(parent.into_bits(), 0xff);