Support fallible iteration methods

[eggyal]

If errors need to be reported during processing, e.g. in the callbacks passed to Structure::each_variant and VariantInfo::construct, one's only real option is to embed compile_error!() macro invocations into the returned TokenStream. However this makes testing a little complex. Would it be possible to provide fallible versions of such methods instead:

impl Structure<'_> {
    pub fn try_each_variant<F, R, E>(&self, f: F) -> Result<TokenStream, E>
    where
        F: FnMut(&VariantInfo<'_>) -> Result<R, E>,
        R: ToTokens,
    { todo!() }
}

and

impl VariantInfo<'_> {
    pub fn try_construct<F, T, E>(&self, func: F) -> Result<TokenStream, E>
    where
        F: FnMut(&Field, usize) -> Result<T, E>,
        T: ToTokens,
    { todo!() }
}

(obviously there would be quite a few such methods across the library).

I think the closest users can currently get is by manually iterating the variants/bindings, collecting into a Result<Vec<_>, _> and then (on the happy path) calling the relevant infallible method that takes from such collected vector—which needlessly allocates and is just a tad tedious.

If this would be acceptable, I'd be happy to submit a PR.