Rename Tick to Event

* Define a tick to be an event with no user data.
* Use the term "event log" for now.
** Reserve the word "entry" for hash entries, and "item" for array items.
** Reserve the word "blockchain" for when the event is a block of something.
** Reserve the word "ledger" for when the event is of a particular type,
   such as transactions.

src/lib.rs

@@ -1,4 +1,4 @@
 #![cfg_attr(feature = "unstable", feature(test))]
-pub mod tick;
+pub mod event;
 extern crate itertools;
 extern crate rayon;