Add runtime support for the upcoming string implementation.

runtime/include/chpl-mem-desc.h:
  Add new memory descriptor types STRING_INIT_DATA and STRING_COPY_REMOTE.

runtime/include/chpl-string.h
runtime/src/chpl-string.c:
  Add new support functions.

  - initString() takes 2 strings, old and new.  If old is longer than
    new, it copies new into old and returns old.  Otherwise, it frees
    old and allocates space for a copy of new.  I'm not crazy about
    this name, so any suggestions are welcome.

  - remoteStringCopy() takes a remote node id, remote address, and
    string length.  It allocates space for the remote string and
    copies the string from the remote locale.

runtime/src/chpl-string-support.c:
  Removed incorrect comments regarding the use of the string literal "".