This PR changes how sidechains are handled.
Before the merge, it was possible to import a chain with lower td and not set it as canonical. After the merge, we expect every chain that we get via InsertChain to be canonical. Non-canonical blocks can still be inserted
with InsertBlockWIthoutSetHead.
If during the InsertChain, the existing chain is not canonical anymore, we mark it as a sidechain and send the SideChainEvents normally.
This PR moves our fuzzers from tests/fuzzers into whatever their respective 'native' package is.
The historical reason why they were placed in an external location, is that when they were based on go-fuzz, they could not be "hidden" via the _test.go prefix. So in order to shove them away from the go-ethereum "production code", they were put aside.
But now we've rewritten them to be based on golang testing, and thus can be brought back. I've left (in tests/) the ones that are not production (bls128381), require non-standard imports (secp requires btcec, bn256 requires gnark/google/cloudflare deps).
This PR also adds a fuzzer for precompiled contracts, because why not.
This PR utilizes a newly rewritten replacement for go-118-fuzz-build, namely gofuzz-shim, which utilises the inputs from the fuzzing engine better.
