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Output the last error message in addition to the final conflict
set, and give a less technical description of the conflict set for the
user message.

An example for the expected output is:

> cabal install --dry-run DPM CurryDB DAV DSH Digit HList MagicHaskeller bamboo
cabal: Could not resolve dependencies:
trying: CurryDB-0.1.1.0 (user goal)
trying: base-4.8.0.0/installed-1b6... (dependency of CurryDB-0.1.1.0)
trying: unix-2.7.1.0/installed-e59... (dependency of process-1.2.3.0/installed-3b1...)
trying: directory-1.2.2.0/installed-b49... (dependency of process-1.2.3.0/installed-3b1...)
Dependency tree exhaustively searched.
I've had most trouble fulfilling the following goals: haskell98 (36850), base
(26359), directory (9599), HTF (5092), bamboo (3788), DPM (3452), unix (3208),
CurryDB (107)

Conflicts are sorted by conflict counts, so high-rated conflicts appear
early in the list. Currently the raw count is printed.

Keep information about the final conflict set and print it in case of
exhaustive search. The original error message is not printed any more.

This commit adds a data structure, 'RetryLog', which is like a difference list
for the 'Progress' type, except that it only supports efficient appends at
failures. Since the solver continually appends logs and calls 'tryWith' while
exploring the search tree, it is important for those operations to be efficient.
Afterwards, the solver converts the 'RetryLog' back to a 'Progress' so that it
can be processed with pattern matching in Log.hs and Message.hs.

Previously, the solver filtered out redundant backjumping messages twice, once
in 'Log.logToProgress' and again in 'Message.showMessages. However,
'showMessages' relied on the backjumping messages to determine where to insert
messages about missing packages. This led to missing "unknown package" messages
(part of issue #3617).

This commit removes the filtering in 'logToProgress', because it was redundant.

This commit is a pure refactoring, no semantic changes. Submitting as separate
PR at @kosmikus request.

Tihs moves `ConflictSet` and `Var` into their own modules.

This commit refactors backjumping so that it uses the 'Progress' type instead of
separate references to a node's children and the conflict set calculated from
those children.

This commit removes references to the solver log that prevented it from being
garbage collected.  It also forces evaluation of the current level and variable
stack in 'Message.showMessages'.

This saves a bit of heap space when running the solver with unlimited
backtracking enabled.

Thanks to Tuncer Ayaz.