EIP-161 State Trie Clearing

Documentation for tests/spurious_dragon/eip161_state_trie_clearing@02aafd78.

Generate fixtures for these test cases for Spurious_dragon with:

fill -v tests/spurious_dragon/eip161_state_trie_clearing --fork Spurious_dragon

Tests for EIP-161: State trie clearing (invariant-preserving alternative).