Implement Clone on the GzDecoder

[Kerollmops]

Hello,

I was wondering if there were a specific reason for the GzDecoder to not implement Clone when the R type implements Clone too.
It doesn't seem to contain complex or unclonable items in it.

Is it just because no one thought about implementing it?
Or to be sure to keep the lib compatible with potential futur versions of the lib?

[Kerollmops]

So I dived into limbo of the crate and apparently the miniz_oxide::core::CompressorOxide doesn't implement Clone and this is the struct that is used by GzDecoder when using the Rust feature flag (I didn't even dive for when the flag is not Rust and the lib use C instead).

[alexcrichton]

I think it'd be great to implement Clone! I'm not entirely sure if the libz library has support for this, however, so we'd need to make sure all the backends support it.