This commit arranges that literal strings will fuse
nicely, by expressing them as an application of build.

* NoRepStr is now completely redundant, though I havn't removed it yet.

* The unpackStr stuff moves from PrelPack to PrelBase.

* There's a new form of Rule, a BuiltinRule, for rules that
  can't be expressed in Haskell.  The string-fusion rule is one
  such.  It's defined in prelude/PrelRules.lhs.

* PrelRules.lhs also contains a great deal of code that
  implements constant folding.  In due course this will replace
  ConFold.lhs, but for the moment it simply duplicates it.