This is a big-ish patch, but it's very straightforward. Under this
clang warning, we're not actually allowed to have a global variable
without a previous extern declaration for it. The cases where we
violated this rule fall into three roughly equal groups:
* Stuff that should have been static.
* Stuff that was global but where the extern was local to some
other C file.
* Stuff that was only global when built for the unit tests, that
needed a conditional extern in the headers.
The first two were IMO genuine problems; the last is a wart of how
we build tests.
Extrainfo documents are now ed-signed just as are router
descriptors, according to proposal 220. This patch also includes
some more tests for successful/failing parsing, and fixes a crash
bug in ed25519 descriptor parsing.
We didn't really have test coverage for these parsing functions, so
I went and made some. These tests also verify that the parsing
functions set the list of invalid digests correctly.
