Support seq and maps (refactor of #129)

[adrianbenavides]

Based on #129

This MR adds support to serialize maps and vectors. Example:

#[derive(Serialize)]
#[serde(rename = "team")]
struct Team {
    people: Vec<Person>,
}

#[derive(Serialize)]
#[serde(rename = "person")]
struct Person {
    name: String,
}

will get serialized as:

<team>
    <people>
        <person>
            <name>Joe</name>
        </person>
        <person>
            <name>Jane</name>
        </person>
    </people>
</team>

Note: the pipeline fails at the same test as the master branch (test_doctype_fail)

[adrianbenavides]

Not sure if this is the expected behavior, but we can't do much to separate the vector items if we don't have any tag. To properly serialize vectors we should use types that wraps a vector such as MyStruct(Vec<String>).