Enum support

[dragonnn]

Hi! I started using your crate on embedded and really like it.
The only think I am missing is enum support, I use it to write structs for spi/i2c devices with often pack some modes settings into they registers.
Some examples:

#[bitfield(u8)]
pub struct PowerCtl {
    #[bits(2)]
    mesure_mode: u8,
    autosleep: bool,
    wakeup: bool,
    #[bits(2)]
    low_noise_mode: u8,
    ext_clk: bool,
    #[bits(1)]
    _unused: u8,
}

#[repr(u8)]
pub enum PowerCtlMesureMode {
    Standby = 0b00,
    Measurement = 0b10,
}

#[repr(u8)]
pub enum PowerCtlLowNoiseMode {
    Normal = 0b00,
    Low = 0b01,
    Ultralow = 0b10,
}

It would be nice if that could become this:

#[bitfield(u8)]
pub struct PowerCtl {
    #[bits(2)]
    mesure_mode: PowerCtlMesureMode,
    autosleep: bool,
    wakeup: bool,
    #[bits(2)]
    low_noise_mode: PowerCtlLowNoiseMode,
    ext_clk: bool,
    #[bits(1)]
    _unused: u8,
}

And in this case when reading a enum value from the struct it should return Option<Enum> so when the registers has a value with isn't allowed by the enum it returns None.
I would love to see that!

[wrenger]

I was thinking about supporting any type that implements From/Into of the underlying bitfield type.

Unfortunately, this is not directly compatible with the raw integer types as they only implement TryFrom/TryInto (which might fail and possibly panic) or casting via as. This means I have to distinguish between ints and custom types and generate different code for them (similar to bools).

Currently I'm quite occupied. So it would take a few weeks before I'm able to implement this. But if you want, you can look into this and contribute it.

[wrenger]

Enums (and other types) that implement From/Into for the bitfield's underlying type are now supported (9b15b05).