Add examples attribute

[ede1998]

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I added a small feature: I tried to add multiple examples to a single struct field but found it quite cumbersome. I had to create a new function for every example values I wanted to add.

Therefore I implemented an attribute examples that works just like example. But instead of providing a single example value, the user can provide anything implementing IntoIterator to return an arbitrary number of examples from a single function.

Example:

#[derive(JsonSchema, Deserialize)]
struct Config {
    #[schemars(examples = "revisions")]
    git_revision: String,
}

fn revisions() -> [&'static str; 3] {
    ["master", "v0.3.0", "5dfcdf8eec66a051ecd85625518cfd13"]
}

This was my first time writing proc-macros so please look out for errors when reviewing (e.g. macro hygiene?).