Add fuzz testing

[howardjohn]

Currently there are a bunch of panics in the unmarshaling logic: https://github.com/go-yaml/yaml/issues?q=is%3Aissue+is%3Aopen+panic

This is the perfect case for fuzzing, which could find many of these bugs quickly.

Resources:

• https://github.com/dvyukov/go-fuzz
• https://github.com/google/oss-fuzz

[dolmen]

See #1028.