Deduplicate Clone by always using match and { }

[ijackson]

This is the RFC MR I proposed in #16.

The overall effect is that we change to always use match and always use { field: ... } syntax, so for example tests::clone_struct::basic's expansion now contains this:

    struct Tuple(u8);
    impl ::core::clone::Clone for Tuple
...
        fn clone(&self) -> Self {
            match self {
                Self { 0: _s_0 } => {
                    Self {
                        0: ::core::clone::Clone::clone(_s_0),
                    }
                }
            }
        }

As I expected, using this approach allows the removal of a lot of code - and so far I've only treated Clone.

If you like this, I'll follow up with similar deduplication for the other traits.

[ijackson]

FTAOD this implements, but only for Clone, these proposals from #16

• "Always using match, unifying enum and struct code"
• "Always using braced syntax"

I haven't tackled "Similarity between Ord, PartialOrd and PartialEq code". I think we should do the non-trait-specific deduplication first.

[ijackson]

Oh, rustfmt. I will rewrite the branch.

[ijackson]

"CI / Test 1.60 on ubuntu-latest () (pull_request) Failing after 20s" CI failure seems to be unrelated to MR contents.

IMO the root cause is not committing a suitable MSRV-compatible lockfile.

[ijackson]

@magiclen do you like the rough shape of this? If so I can proceed with making similar changes to the other impls.