oss-fuzz: update fuzzers to native go fuzzing

[MariusVanDerWijden]

This PR modifies the fuzzers to use native golang fuzzing instead of go-fuzz

[holiman]

Posting what I wrote earlier from discord

I'm thinking that it's two parts really: one pretty simple part of just converting from go-fuzz entrypoints to testing.F entry-points, and another more complicated task of the whole oss-fuzz integration.

And we should really consider if not the fuzzers should be moved from tests/fuzzers/x back into the core codebase.

Previously it made sense to have them separate, because then their go-fuzz import didn't affect the main go.mod, and also the files were excluded from builds, and not part of the public packages. But now that we can put them in _test files, that's not a concern. And having them alongside regular tests and benchmarks make sense, not least since they can reuse many of the same methods.

So I think we should move them. But before we do that, we should figure out if that makes oss-fuzz integration more complicated

[holiman]

Ooh, nice badge:

[MariusVanDerWijden]

Okay the build should be fixed now. I thought we should do this in a two step process:
(1) update the fuzzers to native fuzzing and update oss-fuzz for that
(2) modify the fuzzers to take better advantage of native fuzzing and move them to the package they belong
This PR only addresses (1), but I think its easiest to tackle (2) in a follow-up pr

[holiman]

Looks good enough IMO