Implement Eq and Hash trait for Tag for usage in HashSet.

[VladasZ]

AWS doesn't allow duplicated tags and will fail with error if we try to add it:
<Code>InvalidParameterValue</Code><Message>Duplicate tag key 'tag:tag' specified.</Message>

It would be much more convenient to store tags in HashSet. But rusoto Tag doesn't implement Eq and Hash trait:

/// <p>Describes a tag.</p>
#[derive(Clone, Debug, Default, PartialEq)]
#[cfg_attr(feature = "serialize_structs", derive(Serialize))]
#[cfg_attr(feature = "deserialize_structs", derive(Deserialize))]
pub struct Tag {
    /// <p>The key of the tag.</p> <p>Constraints: Tag keys are case-sensitive and accept a maximum of 127 Unicode characters. May not begin with <code>aws:</code>.</p>
    pub key: Option<String>,
    /// <p>The value of the tag.</p> <p>Constraints: Tag values are case-sensitive and accept a maximum of 255 Unicode characters.</p>
    pub value: Option<String>,
}

Now in my code I have to use vector and write additional checks for duplicated tags. Is it possible to add Eq and Hash implementation for Tag structure?

https://doc.rust-lang.org/std/collections/struct.HashSet.html

HashSet requires that the elements implement the Eq and Hash traits.