Allow specifying custom `from_bits`/`to_bits` implementation for third-party types.

[reitermarkus]

I currently have this in one crate:

/// Clock selection.
#[derive(Debug, Clone, Copy, Eq, PartialEq)]
pub enum ClkSel {
    /// Selects the internal clock.
    Internal,
    /// Selects the external SMA clock input.
    External,
}

impl ClkSel {
    /// Creates a new `ClkSel` variant from a bit representation.
    const fn from_bits(bits: u8) -> Self {
        if bits & 0b1 == 0 {
            Self::Internal
        } else {
            Self::External
        }
    }

    /// Returns the bit representation of the current variant.
    const fn into_bits(self) -> u8 {
        match self {
            Self::Internal => 0,
            Self::External => 1,
        }
    }
}

/// Configuration register.
#[bitfield(u8)]
#[derive(PartialEq, Eq)]
pub struct Cfg {
    #[bits(1)]
    clk_sel: ClkSel,
    dac_resetb: bool,
    dac_sleep: bool,
    dac_txena: bool,
    trf0_ps: bool,
    trf1_ps: bool,
    att0_rstn: bool,
    att1_rstn: bool,
}

Now, I want to move ClkSel into a separate crate, but not move from_bits/into_bits, so I'd need something like this:

const fn clk_sel_from_bits(bits: u8) -> Self {
    if bits & 0b1 == 0 {
        Self::Internal
    } else {
        Self::External
    }
}

const fn clk_sel_into_bits(self) -> u8 {
    match self {
        Self::Internal => 0,
        Self::External => 1,
    }
}

/// Configuration register.
#[bitfield(u8)]
#[derive(PartialEq, Eq)]
pub struct Cfg {
    #[bits(1, from = clk_sel_from_bits, into = clk_sel_into_bits)]
    clk_sel: ClkSel,
    dac_resetb: bool,
    dac_sleep: bool,
    dac_txena: bool,
    trf0_ps: bool,
    trf1_ps: bool,
    att0_rstn: bool,
    att1_rstn: bool,
}

Not sure if it's already possible somehow. I tried using access = na to see if it would be possible to have a different type internally, but this doesn't generate a private getter/setter.

Maybe alternatively, something like access = private would work, or simply not defaulting to pub visibility when no visibility is specified.

[gregorygaines]

@reitermarkus Let me take a look at this