Unlike most other Free Software licences, the MPL explicitly says it
applies only to files with a specific notice:

    1.4. "Covered Software"
	means Source Code Form to which the initial Contributor has attached
	the notice in Exhibit A, the Executable Form of such Source Code
	Form, and Modifications of such Source Code Form, in each case
	including portions thereof.

Unfortunately, this means that copying the MPL text into tree, and
setting the "license" Cargo option, leaves an ambiguous situation.
One might presume that the intent was to actually *use* the MPL for
the whole project, but the legal licence text explicitly rejects that.

Thankfully, according to the licence text, it is not actually
necessary to add the notice to *every* file:

    If it is not possible or desirable to put the notice in a particular
    file, then You may include the notice in a location (such as a LICENSE
    file in a relevant directory) where a recipient would be likely to look
    for such a notice.

So clarify this situation by adding one central explicit copy of the
MPL "Exhibit A" text, and declare it to apply to everything.

Putting it in Cargo.toml puts it next to the "license =" tag.  Another
possibility would be `README.md` but IMO this legal technicality
doesn't really warrant such exposure.

(Putting it in the LICENCE file, as the text itself suggests, would
mean either (i) renaming the verbatim copy of the MPL 2.0 and writing
a new file or (ii) adding it as a rubric to the top of the MPL 2.0
text in LICENCE - resulting in a LICENCE file which is not identical
to the usual MPL-2.0 text file.  Neither of those seem desirable.)