Make it possible to compile the core codebase with stable

[popzxc]

Right now, we are always forced to compile sequencer with nightly compiler.
It's an issue for the projects like era-test-node and foundry-zksync.

In fact, the use of nightly in core is limited (from what I understand, the need for nightly mostly comes from stuff like commitment calculation + proof types), so the nightly functionality can be feature gated.

With that, it should be possible to compile era-test-node/foundry-zksync with stable compiler.