Add support OSS-Fuzz

[darccio]

Mergo was removed from OSS-Fuzz recently because the integration wasn't completed: google/oss-fuzz#9388

Mergo can benefit from fuzz testing, as it's main objective is to merge data, so it seems sensible to make sure there are no strange corner cases lingering in the code (although it's high time for a clean-room implementation).

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