Use "license" instead of "license-file" in Cargo.toml

[briansmith]

Currently we avoid using SPDX expressions to describe the license for at least the following reasons:

• I intend to license the webpki source code under the ISC license but the data files from the Chromium project have a different license. Maybe we shouldn't even be including them in the crate? But it isn't clear if the Cargo.toml license field refers only to the published crate, or also to everything, including files used in the test suite, that is in the repository.

• The spdx.org website says the BSD 3-clause license has a warranty disclaimer that uses the term "COPYRIGHT HOLDER." Yet the license text Google used for Chromium says "COPYRIGHT OWNER" instead. So what SPDX expression covers that?

• The spdx.org website says that the BSD license has a clause "Neither the name of the copyright holder nor the names of its
  contributors may be used to endorse or promote products derived from this software without specific prior written permission." But the Google version says "Neither the name of Google Inc. nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission."

[briansmith]

I noticed when I browse https://github.com/chromium/chromium then GitHub does classify Google's license as BSD 3-clause despite the differences noted above.