Implicit construction of `crow::multipart::message` from `crow::request`

[bgs99]

Would you consider marking crow::multipart::message constructor from crow::request as explicit?

Parsing multipart message is a somewhat computationally expensive process, and currently (#838) leads to excessive memory copying.

I have encountered this problem when writing and using small helpers that extract data from multipart messages - accidentally passing a crow::request there instead of a crow::multipart::message leads to re-parsing of the request.
Marking the constructor as explicit would make this error easily noticeable and less probable.

[gittiver]

Make a PR and make it explicit. If that runs we will take it over.

[gittiver]

PR merged, so can be closed.