Changelog for haxl-2.3.0.0

Changes in version

Add a callgraph reference to 'Env' to record the function callgraph during a computation. The callgraph is stored as an edge list in the Env through the use of withCallGraph and enables users to debug a Haxl computation.

Adds feature to track outgone datasource fetches. This is only turned on for report level greater than 1. The fetches are stored as a running Map in the env ('submittedReqsRef').

Add a new 'w' parameter to 'GenHaxl' to allow arbitrary writes during a computation. These writes are stored as a running log in the Env, and are memoized. This allows users to extract information from a Haxl computation which throws. Our advise is to limit these writes to monitoring and debugging logs.
A 'WriteTree' constructor to maintain log of writes inside the Environment. This is defined to allow O(1) mappend.

Completely rewritten internals to support arbitrarily overlapping I/O and computation. Haxl no longer runs batches of I/O in "rounds", waiting for all the I/O to complete before resuming the computation. In Haxl 2, we can spawn I/O that returns results in the background and computation fragments are resumed when the values they depend on are available. See tests/FullyAsyncTest.hs for an example.
A new PerformFetch constructor supports the new concurrency features: BackgroundFetch. The data source is expected to call putResult in the background on each BlockedFetch when its result is ready.
There is a generic DataSource implementation in Haxl.DataSource.ConcurrentIO for performing each I/O operation in a separate thread.
Lots of cleanup and refactoring of the APIs.
License changed from BSD+PATENTS to plain BSD3.

Some performance improvements, including avoiding quadratic slowdown with left-associated binds.
Documentation cleanup; Haxl.Core is the single entry point for the core and engine docs.
(>>) is now defined to be (*>), and therefore no longer forces sequencing. This can have surprising consequences if you are using Haxl with side-effecting data sources, so watch out!
New function withEnv, for running a sub-computation in a local Env
Add a higher-level memoization API, see 'memo'
Show is no longer required for keys in cachedComputation
Exceptions now have Eq instances