[storage] Allow one variant enum to derive SpreadLayout

[tash-2s]

I have an enum that has only one variant, and I want to store the enum value in the storage. I might add new variants later in development, so the enum is not useless.

The current implementation doesn't allow to derive the SpreadLayout trait for one variant enums, and I fixed it on this PR.

Sample code

#![cfg_attr(not(feature = "std"), no_std)]

use ink_lang as ink;

#[derive(
    Copy,
    Clone,
    scale::Encode,
    scale::Decode,
    ink_storage::traits::SpreadLayout,
    ink_storage::traits::PackedLayout,
)]
#[cfg_attr(
    feature = "std",
    derive(scale_info::TypeInfo, ink_storage::traits::StorageLayout)
)]
pub enum MyEnum {
    A,
}

#[ink::contract]
mod enum_test {
    use super::MyEnum;

    #[ink(storage)]
    pub struct EnumTest {
        value: MyEnum,
    }

    impl EnumTest {
        #[ink(constructor)]
        pub fn new() -> Self {
            Self { value: MyEnum::A }
        }

        #[ink(message)]
        pub fn get(&self) -> MyEnum {
            self.value
        }
    }
}

Without this change, I get this error.

$ cargo +nightly contract build
 [1/5] Building cargo project
    Updating crates.io index
   Compiling enum_test v0.1.0 (/private/var/folders/zn/l2f569z56vnghtt524x1mv6w0000gn/T/cargo-contract_FM50JF)
error: proc-macro derive panicked
  --> /snip/enum_test/lib.rs:10:5
   |
10 |     ink_storage::traits::SpreadLayout,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: message: can only operate on enums

[Robbepop]

Thank you for the PR. All in all LGTM.

[codecov-commenter]

Codecov Report

Merging #942 (78b31e4) into master (beaf989) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #942      +/-   ##
==========================================
- Coverage   82.88%   82.86%   -0.03%     
==========================================
  Files         177      177              
  Lines        8058     8059       +1     
==========================================
- Hits         6679     6678       -1     
- Misses       1379     1381       +2     

Impacted Files	Coverage Δ
crates/storage/derive/src/spread_layout.rs	98.50% <100.00%> (+0.02%)	⬆️
...ates/storage/src/collections/hashmap/fuzz_tests.rs	97.87% <0.00%> (-2.13%)	⬇️

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[tash-2s]

Thank you for your review. CI passed.

[Robbepop]

LGTM
Thank you a lot for writing the test case!
This PR really is an improvement. :)