Add .bazelignore for aocgen, go, and rust

These are not yet built using Bazel, so they should probably be
ignored. More specifically, `aocgen` contains templates for BUILD
files, but they should not be considered actual packages. `go` and
`rust` are ignored so that Bazel does not happen to pick up anything
there (though I eventually intend to build these directories using
Bazel as well).

Author: saser

.bazelignore

@@ -0,0 +1,3 @@
+aocgen
+go
+rust