Copyright identifiers for Profian Inc

[tgross35]

All source have SPDX-FileCopyrightText: 2022 Profian Inc. <opensource@profian.com>, which isn't stated in any other rustcrypto files. Is this intentional?

@npmccallum It looks like this came from your initial code dump, I'm not what the ownership rights are intended to be.

[tarcieri]

It's pretty uncommon for our code to include copyright identifiers in each file. The Apache 2.0 license does recommend including them, however.