ChainConfig hooks for subnet-evm

[darioush]

Rationale

To use libevm in subnet-evm, and allow for use of custom forks in the params package,
we could extend the ChainConfig hooks to include:

	CheckCompatible(newcfg *ChainConfig, headNumber *big.Int, headTimestamp uint64) *ConfigCompatError
	CheckConfigForkOrder() error
	Description() string

Implementation

draft implementation: https://github.com/ava-labs/go-ethereum/pull/11/files#diff-10ff00a15ef58da7485fedeca3906c73cb236fde4b143a9d61601c63cbf1ffe8