feat: add #[borsh(crate = ...)] item-level attribute

[dj8yfo]

resolves #95
Need for this discovered in scope of work in near-sdk-rs update

[dj8yfo]

Sample crate with this attribute tested.

[dj8yfo]

parsing error was silenced here, allowing to declare following value for the attribute without any syn::Error and returning None

#[derive(BorshSerialize, BorshDeserialize, PartialEq, Debug)]
#[borsh(init={strange; blocky})]
struct A {
    lazy: Option<u64>,
}

#[derive(BorshSerialize, BorshDeserialize, PartialEq, Debug)]
#[borsh(init=42)]
struct A {
    lazy: Option<u64>,
}

[dj8yfo]

this test doesn't change due to impl-ToTokens-for-Option

[frol]

Overall, looks great to me!

[frol]

Can you help me understand why we need to parse something completely unrelated here?

[dj8yfo]

If we comment out these 115-117 highlighted lines, we get a failure in test with following message:

---- internals::attributes::item::tests::test_init_function_with_use_discriminant_with_crate stdout ----
thread 'internals::attributes::item::tests::test_init_function_with_use_discriminant_with_crate' panicked at 'called `Result::unwrap()` on an `Err` value: Error("expected `,`")', borsh-derive/src/internals/attributes/item/mod.rs:412:20

The recommended parsing helper
parses the args init = initialization_method, crate = "reexporter::borsh", use_discriminant=true of #[borsh(init = initialization_method, crate = "reexporter::borsh", use_discriminant=true)] with following
code somewhere down the way:

pub(crate) fn parse_nested_meta(
    input: [ParseStream](https://docs.rs/syn/latest/src/syn/parse.rs.html#223),
    mut logic: impl FnMut([ParseNestedMeta](https://docs.rs/syn/latest/src/syn/meta.rs.html#163-166)) -> Result<()>,
) -> Result<()> {
    loop {
        let path = input.call(parse_meta_path)?;
        logic(ParseNestedMeta { path, input })?;
        if input.is_empty() {
            return Ok(());
        }
        input.parse::<Token![,]>()?;
        if input.is_empty() {
            return Ok(());
        }
    }
}

It expects to completely consume input ParseStream<'a>, without skipping anything.
If we comment out let _value_expr: Expr = meta.value()?.parse()?; it returns an error input.parse::<Token![,]>()?; on a new loop iteration on a = punctuation, presumably originating here.