This commit serves as the basis for further `v5` developments. It will introduce some API-breaking changes, especially to the way tokens are validated. This will allow us to provide some long-wanted features with regards to the validation API. We are aiming to do this as smoothly as possible, however, with any major version. please expect that you might need to adapt your code.

The actual development will be done in the course of the next week, if time permits. It will be done in seperate PRs that will use this PR as a base. Afterwards, we will probably merge this and release an initial 5.0.0-alpha1 or similar.

This PR removes the old legacy standard claims, which have been deprecated since the beginning of the `v4` module in favor of the newer `RegisteredClaims`. Removing them before any further changes to the validation API is quite useful, as less code needs to be adapated.

* New Validation API

Some guidelines in designing the new validation API

* Previously, the `Valid` method was placed on the claim, which was always not entirely semantically correct, since the validity is concerning the token, not the claims. Although the validity of the token is based on the processing of the claims (such as `exp`). Therefore, the function `Valid` was removed from the `Claims` interface and the single canonical way to retrieve the validity of the token is to retrieve the `Valid` property of the `Token` struct.
* The previous fact was enhanced by the fact that most claims implementations had additional exported `VerifyXXX` functions, which are now removed
* All validation errors should be comparable with `errors.Is` to determine, why a particular validation has failed
* Developers want to adjust validation options. Popular options include:
  * Leeway when processing exp, nbf, iat
  * Not verifying `iat`, since this is actually just an informational claim. When purely looking at the standard, this should probably the default
  * Verifying `aud` by default, which actually the standard sort of demands. We need to see how strong we want to enforce this
* Developers want to create their own claim types, mostly by embedding one of the existing types such as `RegisteredClaims`.
  * Sometimes there is the need to further tweak the validation of a token by checking the value of a custom claim. Previously, this was possibly by overriding `Valid`. However, this was error-prone, e.g., if the original `Valid` was not called. Therefore, we should provide an easy way for *additional* checks, without by-passing the necessary validations

This leads to the following two major changes:

* The `Claims` interface now represents a set of functions that return the mandatory claims represented in a token, rather than just a `Valid` function. This is also more semantically correct.
* All validation tasks are offloaded to a new (optional) `validator`, which can also be configured with appropriate options. If no custom validator was supplied, a default one is used.

Co-authored-by: Micah Parks <66095735+MicahParks@users.noreply.github.com>

This PR adds further documentation to the validator and does an additional cleanup to make the VerifyXXX functions more managable.

Co-authored-by: Michael Fridman <mf192@icloud.com>