core/vm, cmd/evm: unexport errors, remove err-to-code mapping, new te…

…sts, last fix

Changes include:
- unexporting errors, removing error-to-code mapping
- new test vectors from execution-spec-tests v [email protected]
- remaining fix from @shemnon in MariusVanDerWijden#56
- core/vm: address review-comments - simplify code
- cmd/evm: move eofdump/eofparse into `evm` binary
- Also makes eofdump read from stdin if hex is not provided, and makes the output of eofdump a bit more eye-friendly.
- core/vm: refactor check in eof control-flow validation
- core/vm: refactor some control flow checks for readability