request/response verification helpers

[cmazakas]

In some cases, it's easiest to determine the validity of a message once the headers are complete (i.e. the closing \r\n has been parsed).

To aid users in checking if their messages are rfc-compliant, we introduce a prototype API:

result<void> validate(response_view const&);
result<void> validate(request_view const&);

Validity Requirements (incomplete):

• A client MUST NOT generate a 100-continue expectation in a request that does not include content.