Name and order the memory core code

The names of the memory core that only one of its parts uses start with
that part: the day helpers of the journal, the record decoder of the
tree, the message kinds and check of the log, and the view file of the
memory. The types of the core start with a capital letter: Journal,
Position, Records, Syncer, and NodeKey.

The fake summarizer of the tests is in summarizer_test.go, because most
tests use it. The test helpers of the tree, the log, the syncer, and the
replay test start with their part. The declarations of the core files
follow the order of the go-code skill. The behavior does not change.

Issue: 947bddb