Take into account deny_unknown_fields attribute when deserialzie struct from sequence

[Mingun]

This PR expands scope of deny_unknown_fields container attribute to sequences, making behaviour consistent when struct deserialized from a map or from a sequence:

#[derive(Deserialize)]
struct A {
    number: i32,
    text: String,
}

could be deserialized from

{
    "number": 1234,
    "text": "hello",
    "dont": 345.234,
    "really": [],
    "care": true
}

and from

[
    1234,
    "hello",
    345.234,
    [],
    true
]

The

#[derive(Deserialize)]
#[serde(deny_unknown_fields)]
struct A {
    number: i32,
    text: String,
}

could be deserialized only from

{
    "number": 1234,
    "text": "hello"
}

and from

[
    1234,
    "hello"
]

Closes #2432

[dtolnay]

Thanks for the PR!

#[derive(Deserialize)]
struct A {
    number: i32,
    text: String,
}

...

could be deserialized from

[
    1234,
    "hello",
    345.234,
    [],
    true
]

It is not clear to me that this is desirable, or the behavior that most users would expect to get here from derived Deserialize impls by default.

I left a suggestion in #2432 (comment) -- I think I would prefer if this were pursued in a different library.