Template Haskell: improve lifting for strings

Template Haskell: improve lifting for strings

When you have a (\s::String -> ....[| s |]....), the string
's' is lifted.  We used to get a chain of single-character
Cons nodes, correct but lots and lots of code.

This patch arranges to optimise that to a string literal. It does
so in two places:
  a) In TcExpr, if we know that s::String, we generate liftString directly
  b) In DsMeta, if we find a list of character literals, we convert to
     a string.  This catches a few cases that (a) does not

There an accompanying  patch in the template-haskell package,
adding Language.Haskell.TH.Syntax.liftString