core/types: pin code generators versions when alias types are supported

[qdm12]

We use two code generators to generate codec functions for JSON and RLP codecs:

1. github.com/fjl/gencodec for JSON - done in #856
2. github.com/ava-labs/libevm/rlp/rlpgen for RLP (code forked from geth)

The support for type aliases is ongoing as follows:

1. https://github.com/fjl/gencodec/pull/25
2. https://github.com/ava-labs/libevm/pull/154
3. https://github.com/fjl/gencodec/pull/27

Once these are done, we should:

1. Upgrade libevm to the version containing this new feature
2. (done) Pin fjl/gencodec version to the version containing this new feature, using tools.go + go mod tidy
3. (half-done) Add a CI check to ensure these generated files are kept in sync (similar to how it is done with mockgen)

[ARR4N]

https://github.com/ava-labs/libevm/pull/154 isn't blocked by a Geth sync; it's unblocked by https://github.com/ava-labs/libevm/pull/155 and https://github.com/ava-labs/libevm/pull/156.

[qdm12]

Blocked by https://github.com/fjl/gencodec/pull/26

[qdm12]

ava-labs/libevm#154 isn't blocked by a Geth sync; it's unblocked by ava-labs/libevm#155 and ava-labs/libevm#156.

Awesome! 💯 Should we then do another libevm release soon-ish so I can incorporate this in our coreth libevm branch? 🤔