This unifies several of the places in CSA that convert various
string kinds (cons, thin, sliced) to direct strings
(sequential, external).

A couple of spots remain with duplicate code, but most of these are
more difficult to unify due to specific optimizations.

BUG=

Review-Url: https://codereview.chromium.org/2744263002
Cr-Commit-Position: refs/heads/master@{#43848}