Enum spread allocate

[forgetso]

This is my attempt to identify when an enum has derived Default from its representation as a synstructure for #1042.

For exmaple, an enum like this would be allowed:

#[derive(Default)]
enum Enum { A, B, #[default] C }

But this one would fail:
enum Enum { A, B, C }

I don't know how to do this without using the Default derive. If you create the default value using impl, the synstructure, contains no information about the default. Additionally, you cannot pass impl into the synstructure::Structure::new.

    pub enum Enum {
        A,
        B,
        C,
    }

    impl Default for Status {
        fn default() -> Self { Enum::C }
    }

[forgetso]

Also, I've been running rustfmt on this but it seems to format it differently from your CI rustfmt.

[HCastano]

@forgetso thanks for the PR! I'm gonna put this on-ice since we're still unsure about the direction we want to take with SpreadAllocate in general.

[paritytech-cicd-pr]

🦑 📈 ink! Example Contracts ‒ Changes Report 📉 🦑

These are the results when building the examples/* contracts from this branch with cargo-contract 0.17.0-86510f8 and comparing them to ink! master:

	Δ Optimized Size	Δ Used Gas	Total Optimized Size	Total Used Gas
accumulator			1.44 K
adder			2.94 K
contract-terminate		+14_926	1.31 K	215_704
contract-transfer		+14_305	7.98 K	15_704
delegator		+41_047	7.63 K	51_343
dns		+31_959	10.20 K	47_112
erc1155		+46_413	27.50 K	94_224
erc20		+37_098	9.80 K	47_112
erc721		+90_930	13.70 K	125_632
flipper		+15_357	1.76 K	15_704
incrementer		+15_364	1.64 K	15_704
multisig		+46_824	26.79 K	103_580
proxy		+29_614	3.81 K	32_243
rand-extension		+9_050	5.12 K	15_704
subber			2.96 K
trait-erc20		+36_453	10.10 K	47_112
trait-flipper		+15_362	1.53 K	15_704
trait-incrementer		+30_728	1.62 K	31_408

Link to the run | Last update: Tue Feb 8 21:14:16 CET 2022

[HCastano]

Closing since this will be made obsolete with #1217. Thanks for the PR though!