Without this, if it fails, the only information printed is useless:

    Preparing a sysroot for Miri (target: x86_64-unknown-linux-gnu)...
    fatal error: failed to build sysroot; run `cargo miri setup` to see the error details

Use syn::Error's combine() API instead of Vec<syn::Error>

This reverts commit e77db40.

    error: consider adding a `;` to the last statement for consistent formatting
       --> serde_derive/src/internals/ast.rs:161:13
        |
    161 |             seen_untagged = variant.attrs.untagged()
        |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: add a `;` here: `seen_untagged = variant.attrs.untagged();`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#semicolon_if_nothing_returned
        = note: `-D clippy::semicolon-if-nothing-returned` implied by `-D clippy::pedantic`

    error: consider adding a `;` to the last statement for consistent formatting
       --> serde_derive/src/internals/ast.rs:159:17
        |
    159 | ...   cx.error_spanned_by(&variant.ident, "all variants with the #[serde(untagged)] attribute must be placed at the end of the enum")
        |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: add a `;` here: `cx.error_spanned_by(&variant.ident, "all variants with the #[serde(untagged)] attribute must be placed at the end of the enum");`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#semicolon_if_nothing_returned

…alization

Reuse a single ContentRefDeserializer throughout untagged enum deserialization

Previously if someone wrote an enum containing:

- `A` (untagged)
- `B` (tagged)
- `C` (tagged)
- `D` (untagged)
- `E` (tagged)
- `F` (untagged)

serde_derive would produce errors referring to B and E only, saying
you're supposed to put untagged variants at the end. The choice of B and
E for this error doesn't make a lot of sense because in order to resolve
the issue, the user must either:

- move A and D down

or:

- move B, C, and E up.

This commit changes the error to appear on A and D instead.