fix: stop committing bin/, which is build output that looks like source

Eclipse/Buildship writes a duplicate copy of every .kt file into bin/main and
bin/test for the pure-JVM modules. Fourteen of them reached the previous commit,
because a directory full of Kotlin files is the one kind of build output that
does not look like build output in a `git status`.

Removed from the index and ignored. Nothing was lost — every file under bin/ was
a copy of one under src/.

.gitignore

@@ -17,6 +17,10 @@ play-service-account*.json
 # Gradle and Android build output
 .gradle/
 build/
+# Eclipse/Buildship writes duplicate sources here for the pure-JVM modules. It
+# is build output that looks exactly like source, and it slipped into a commit
+# once before this line existed.
+bin/
 captures/
 .cxx/
 *.apk