AVRO-3575: Rust: add fuzz harness

[evanrichter]

The cargo-fuzz harness can be run with cargo +nightly fuzz run roundtrip from the lang/rust/avro folder.

The second commit fixes 1 panic, 1 oom, and 1 infinite loop that the fuzzer found.

Jira

• https://issues.apache.org/jira/browse/AVRO-3575

Tests

• My PR adds fuzz testing
• The code fixes do not break cargo test
• The code fixes pass fuzz testing and fix all panic/oom/timeout

Commits

• My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • No new functionality

[martin-g]

Thank you for the contribution and the fixes, @evanrichter !