Declare structs for types with more than a single `ReadArgs` argument

[cmyr]

Currently if a type has multiple read arguments, we pass them in as a tuple, but this makes it very easy to lose track of them, especially if the arguments are of the same type. It would be nice if instead of this, we declared a struct with named fields, and used that.

In this world,

#[read_args(number_of_h_metrics: u16, num_glyphs: u16)]
table Hmtx {
    // ...
}

would produce:

pub struct HmtxArgs {
    pub number_of_h_metrics: u16,
    pub num_glyphs: u16,
}

impl ReadArgs for Hmtx {
    type Args = HmtxArgs;
}

impl<'a> FontReadWithArgs<'a> for Hmtx<'a> {
    fn read_with_args(data: FontData<'a>, args: &HmtxArgs) -> Result<Self, ReadError> {
        let HmtxArgs { number_of_h_metrics, num_glyphs } = *args;
        // ..
    }
}

instead of,

impl ReadArgs for Hmtx {
    type Args = (u16, u16);
}

impl<'a> FontReadWithArgs<'a> for Hmtx<'a> {
    fn read_with_args(data: FontData<'a>, args: &(u16, u16)) -> Result<Self, ReadError> {
        let (number_of_h_metrics, num_glyphs) = *args;
    }
}

which should be much easier to understand.

[rsheeter]

Ideally I would like to see us generate that struct, though I wouldn't view that as a hard requirement

[cmyr]

My intention was also for us to generate the args struct.