Carries out some remaining tidy-ups remaining after PR 15141:

- split ValidationState into TxValidationState and BlockValidationState (commit from ajtowns)
- various minor code style tidy-ups to the ValidationState class
- remove the useless `ret` parameter from `ValidationState::Invalid()`
- remove the now unused `first_invalid` parameter from `ProcessNewBlockHeaders()`
- remove the `fMissingInputs` parameter from `AcceptToMemoryPool()`, and deal with missing inputs the same way as other errors by using the `TxValidationState` object.

Tip for reviewers (thanks ryanofsky!): The first commit ("[validation] Add CValidationState subclasses" ) is huge and can be easier to start reviewing if you revert the rote, mechanical changes:

Substitute the commit hash of commit "[validation] Add CValidationState subclasses" for <CommitHash> in the commands below.

```sh
git checkout <CommitHash>
git grep -l ValidationState | xargs sed -i 's/BlockValidationState\|TxValidationState/CValidationState/g'
git grep -l ValidationResult | xargs sed -i 's/BlockValidationResult\|TxValidationResult/ValidationInvalidReason/g'
git grep -l MaybePunish | xargs sed -i 's/MaybePunishNode\(ForBlock\|ForTx\)/MaybePunishNode/g'
git diff HEAD^
```

After that it's possible to easily see the mechanical changes with:

```sh
git log -p -n1 -U0 --word-diff-regex=. <CommitHash>
```

laanwj:
  ACK 3004d5a12d09d94bfc4dee2a8e8f2291996a4aaf
amitiuttarwar:
  code review ACK 3004d5a12d09d94bfc4dee2a8e8f2291996a4aaf. Also built & ran tests locally.
fjahr:
  Code review ACK 3004d5a12d09d94bfc4dee2a8e8f2291996a4aaf . Only nit style change and pure virtual destructor added since my last review.
ryanofsky:
  Code review ACK 3004d5a12d09d94bfc4dee2a8e8f2291996a4aaf. Just whitespace change and pure virtual destructor added since last review.