Follow ActiveSupport's instrumentation format

[ericproulx]

We should follow the pattern event.library.

Instead of

SERIALIZABLE_HASH_NOTIFICATION = 'render.bright_serializer.serializable_hash'
SERIALIZED_JSON_NOTIFICATION = 'render.bright_serializer.serializable_json'

we should

SERIALIZABLE_HASH_NOTIFICATION = 'serializable_hash.bright_serializer'
SERIALIZED_JSON_NOTIFICATION = 'serializable_json.bright_serializer'