remove the Obj.magic from the string.ml implementation

It is important not to assume that String.t and Bytes.t will always
share the same representation. Using Obj.magic to convert between
functions would give a very bad example to users considering
a migration, which are very quick to imitate any moral turpitude found
in the standard library.

An unfortunate consequence of the change is the duplication of
String.concat code; other designs would be possible to share more
implementation details between Bytes and String (eg. defined
factorized operations on both in a shared internal module), but if we
consider that String representation may evolve in the future this
coincidence of implementation is really a temporary coindence rather
than an definitive duplication.

I checked that all the small functions introduced are marked as
inlinable. In the case of coercions like this, we could even have the
compiler recognize eta-expansions of the identity function and turn
them into simple rebindings.

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15061 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02