Replace #[start] with extern fn main

Drop #[lang = "eh_personality"] from no-std test

    warning: item uses `packed` representation without ABI-qualification
       --> test_suite/tests/test_gen.rs:774:5
        |
    772 |       #[repr(packed)]
        |              ------ `packed` representation set here
    773 |       #[allow(dead_code)]
    774 | /     struct Packed {
    775 | |         x: u8,
    776 | |         y: u16,
    777 | |     }
        | |_____^
        |
        = warning: unqualified `#[repr(packed)]` defaults to `#[repr(Rust, packed)]`, which has no stable ABI
        = help: qualify the desired ABI explicity via `#[repr(C, packed)]` or `#[repr(Rust, packed)]`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#repr_packed_without_abi
    note: the lint level is defined here
       --> test_suite/tests/test_gen.rs:5:9
        |
    5   | #![deny(warnings)]
        |         ^^^^^^^^
        = note: `#[warn(clippy::repr_packed_without_abi)]` implied by `#[warn(warnings)]`

    warning: item uses `packed` representation without ABI-qualification
       --> test_suite/tests/test_gen.rs:919:1
        |
    918 |   #[repr(packed)]
        |          ------ `packed` representation set here
    919 | / pub struct RemotePacked {
    920 | |     pub a: u16,
    921 | |     pub b: u32,
    922 | | }
        | |_^
        |
        = warning: unqualified `#[repr(packed)]` defaults to `#[repr(Rust, packed)]`, which has no stable ABI
        = help: qualify the desired ABI explicity via `#[repr(C, packed)]` or `#[repr(Rust, packed)]`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#repr_packed_without_abi

    warning: item uses `packed` representation without ABI-qualification
       --> test_suite/tests/test_gen.rs:927:1
        |
    925 |   #[repr(packed)]
        |          ------ `packed` representation set here
    926 |   #[serde(remote = "RemotePacked")]
    927 | / pub struct RemotePackedDef {
    928 | |     a: u16,
    929 | |     b: u32,
    930 | | }
        | |_^
        |
        = warning: unqualified `#[repr(packed)]` defaults to `#[repr(Rust, packed)]`, which has no stable ABI
        = help: qualify the desired ABI explicity via `#[repr(C, packed)]` or `#[repr(Rust, packed)]`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#repr_packed_without_abi

    warning: item uses `packed` representation without ABI-qualification
       --> test_suite/tests/test_gen.rs:937:1
        |
    936 |   #[repr(packed)]
        |          ------ `packed` representation set here
    937 | / pub struct RemotePackedNonCopy {
    938 | |     pub a: u16,
    939 | |     pub b: String,
    940 | | }
        | |_^
        |
        = warning: unqualified `#[repr(packed)]` defaults to `#[repr(Rust, packed)]`, which has no stable ABI
        = help: qualify the desired ABI explicity via `#[repr(C, packed)]` or `#[repr(Rust, packed)]`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#repr_packed_without_abi

    warning: item uses `packed` representation without ABI-qualification
       --> test_suite/tests/test_gen.rs:945:1
        |
    943 |   #[repr(packed)]
        |          ------ `packed` representation set here
    944 |   #[serde(remote = "RemotePackedNonCopy")]
    945 | / pub struct RemotePackedNonCopyDef {
    946 | |     a: u16,
    947 | |     b: String,
    948 | | }
        | |_^
        |
        = warning: unqualified `#[repr(packed)]` defaults to `#[repr(Rust, packed)]`, which has no stable ABI
        = help: qualify the desired ABI explicity via `#[repr(C, packed)]` or `#[repr(Rust, packed)]`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#repr_packed_without_abi

- deserialization of flatten unit variant is possible
- serialization of such variant gives Err("can only flatten structs and maps (got an enum)")