release 0.8.0

- major refactoring to the database APIs for safer parallel
  processing (#78, #82):
  - To set an input, you now write `db.query_mut(Query).set(...)`,
    and you must declare your database as `mut`.
  - To fork a thread, you now write `db.snapshot()`, which acquires
    a read-lock that is only released when the snapshot is dropped
    (note that this read-lock blocks `set` from occuring on the main
    thread).
  - Therefore, there can only be one mutable handle to the
    database; all other handles are snapshots. This eliminates a variety
    of complex and error-prone usage patterns.
- introduced the `salsa_event` callback that can be used for logging
  and introspection (#63)

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "salsa"
-version = "0.7.0"
+version = "0.8.0"
 authors = ["Niko Matsakis <niko@alum.mit.edu>"]
 edition = "2018"
 license = "Apache-2.0 OR MIT"