coverage: disable the concept of "CI failures"

code coverage may be interesting to some people as an informational
update, but it's really fragile and sometimes obscure, and overall we
would really like to NOT have most PRs reporting a red X in the CI
overview, when all project/unittests succeeded but codecov decides that
by some inscrutbale metric, coverage by % has dropped.

Elegant refactorings are penalized, because removing lines of code or
rewriting them to be more compact, means the overall percentage of
covered code is "less", even though no uncovered code got added.

Even worse, the coverage reports often erroneously complain that a PR
has "added lines #L<num> - L<num> were not covered by tests" even though
github helpfully points out they are "Unchanged files with check
annotations". Or more generally, codecov claims that coverage has
dropped in code which the PR can't touch.

The whole thing is just too much of a source of trouble. So, configure
codecov to consider all PRs as successful no matter what. It is still
welcome to leave informational comments, though.

.github/codecov.yml

@@ -0,0 +1,8 @@
+coverage:
+  status:
+    project:
+      default:
+        informational: true
+    patch:
+      default:
+        informational: true