JumpTable customization

[darioush]

Rationale

Subnet-evm and coreth both include modifications to the jump table which activate at different forks.

In addition to type-safety and being mindful of package import order, an additional challenge here is that defining new opcodes would likely need access to unexported types & identifiers from the core/vm package.
(See this PR for examples of extensions used in coreth)

Implementation

A hacky implementation (non-type safe, and implements extensions in the core/vm package which is not ideal): https://github.com/ava-labs/go-ethereum/pull/11/files#diff-9c1fde428c6130ced0ba724d0f195562f64913291362ea0fcb6cefcdd6e16d42