Use MultiGzDecoder to support multi-member gzip files

[nirosys]

Issue #, if available: #126

Description of changes:

This PR changes the GzDecoder used for decompressing gzip'd inputs to a MultiGzDecoder in order to handle multi-member gzip files.

Pre-PR (using homebrew'd ion-cli):

glitch@147dda5e5395 ~/C/ion-cli> ion dump user-data.ion.gz | ion beta count
1213

Post-PR:

glitch@147dda5e5395 ~/C/ion-cli> cargo run --release dump user-data.ion.gz | ion beta count
    Finished release [optimized] target(s) in 0.19s
     Running `target/release/ion dump user-data.ion.gz`
2541
glitch@147dda5e5395 ~/C/ion-cli> 

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.