Fix (wrong) clang 3.4 warning (happens on Mavericks)

* Might be somewhat related to http://fr.it-usenet.org/thread/11425/24401/
* Seems due to the usage of both constexpr and inline.
* We simply inline the functions as they are so trivial.

In file included from .../mozart2/vm/vm/main/serializer.cc:25:
In file included from .../mozart2/vm/vm/main/mozart.hh:28:
In file included from .../mozart2/vm/vm/main/mozartcore.hh:28:
In file included from .../mozart2/vm/vm/main/mozartcore-decl.hh:36:
In file included from .../mozart2/vm/vm/main/typeinfo-decl.hh:35:
.../mozart2/vm/vm/main/lstring-decl.hh:167:34: warning: inline function 'mozart::mut::LString<unsigned char>::slice' is not defined
      [-Wundefined-inline]
  inline constexpr const LString slice(nativeint from) const;
                                 ^
.../mozart2/vm/vm/main/bytestring.hh:127:44: note: used here
  LString<unsigned char> haystack = _bytes.slice(fromOffset);
                                           ^
1 warning generated.