This brings %DebugPrint(string) closer to %DebugPrint(object) by also
including the pointer, object kind, and RO/old space. Especially the
pointer can be useful while debugging.

One could consider going even further end printing full details of
the string object, e.g. first and second pointers for cons strings.

Before:

 $ out/debug/d8 --allow-natives-syntax -e '%DebugPrint("abc");'
 DebugPrint: #abc
 0x263f080402cd: [Map] in ReadOnlySpace
 [...]

After:

 $ out/debug/d8 --allow-natives-syntax -e '%DebugPrint("abc");'
 DebugPrint: 0xa830824ffe1: [String] in OldSpace: #abc
 0xa83080402cd: [Map] in ReadOnlySpace
 [...]

Drive-by: Document string printing functions.
Drive-by: Use PrintUC16 in spots that don't want a full debug print.

Bug: v8:10581
Change-Id: Ided59047b9c3edc1830ce7721376dddfd24fad1d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2228509Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68136}