[ fix ] Fixed whitespace in test/succeed/QuoteContext.agda
[ Cabal ] Added cpphs flag

We use cpphs by default but we are having some difficult issues. The
flag allows to use the default C preprocessor instead of cpphs.
Merge branch 'maint-2.4.2'

Conflicts:
	configure.ac
[ sharing ] ignoreSharing missing at places in instance search
[ instance ] allow metas in instance target if they are fixed by types

Typically: level metas
[ instance ] check candidates for equality in local TC state
[ Serialize ] Benchmarking compression time. Huffman only saves 50%!

In trade for 30% more disk space.  It is worth keeping this
optimization: saves 5s of 220s on std-lib.
[ Serialize ] Benchmark data for std-lib (Andreas' laptop).
[ Serialize ] Specializing icodeX to Integer, String, Double, Node.

Mostly neutral to performance (slightly beneficial).
[ Serialize ] Benchmark data for std-lib (Andreas' laptop).
[ Serialize ] INLINE icode<nnn> does not help.
[ Serialize ] Removed -O2, as it does not speed up serializer.
[ Benchmark ] Added subphases Compress, BinaryEncode of Serialize.
[ comment ] Typo fixed in haddocumentation.
[ Serialize ] Benchmark data for std-lib (Andreas' laptop).
[ new ] Utils.IORef, base<4.6 compatibility for Data.IORef.
[ Serialize ] Prettier printing of statistics.
[ refactor ] use fromMaybe.
[ Serialize ] Reuse statistics for Node, Double, Integer, String.
[ Serialize ] Entry reuse statistics for hash maps in library-test.