Move shared helpers to files of their own

Move shared helpers to files of their own

writeFileAtomic moves from memory.go to file.go with no change, because
the token file of the login uses it too, and a helper in the file of its
first user hides the dependency of the other. The test clock moves from
log_test.go to clock_test.go for the same reason: the memory, tree, and
replay tests use it only as a clock.

A search of every cross-file use finds no other helper with an unrelated
user. The other shared declarations are parts of the memory core or of
the model client that compose for one purpose.

Issue: b14d537