Changelog for haxl-2.4.0.0

Changes in version 2.4.0.0

Added fetchBatchId to FetchStats
Profiling now tracks full stacks and links each label to memos/fetches
Adds FetchDataSourceStats used to log stats/profiling data returned from datasources. This is stored in statsRef like any other Stats.
Report flag was changed from sequential numbers to bitmask.
Add ReportExceptionLabelStack flag to include label stack in HaxlException.

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