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Suggested by Heinrich Apfelmus on the libraries list.

Part of trac #5529. We also now import the constructors in various
modules that use them in FFI decls.

An expression like
    assert False (throw e)
should throw the assertion failure rather than e

Due to a wrong order of tests, values near the normalised/denormalised
border were sent down the wrong branch. That led to rounding twice and
picking the wrong neighbour for some values.

fromRat' no longer uses scaleRat, if we're confident nobody uses that,
we could remove it.

We don't use iconv or libcharset on Windows, but if configure finds
them then it can cause problems. So we don't even try looking if
we are on Windows.

See http://www.haskell.org/pipermail/cvs-ghc/2011-September/065980.html

Patch from Bill Tutt.

Avoid a few unnecessary gcds to speed up reading.

The denominator is always 1, so there's no need for gcd.

It was going via Integer before

They were in GHC.Word, but are also used in GHC.Int