I extracted following class member functions to string.cc
* String
* SeqString
* SeqOneByteString
* SeqTwoByteString
* ConsString
* ThinString
* SlicedString
* ExternalString
* FlatStringReader
* ConsStringIterator

Declaration of all above class are in string.h.

This patch makes compile time of objects.cc from 20.6s to 19.2s on Z840 Linux.

Bug: v8:7629
Change-Id: If74b868b3a3d9a1df2887f82e2557da43ad221f0
Reviewed-on: https://chromium-review.googlesource.com/c/1446342Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59235}