Moves all but conversions.*, hash-seed-inl.h and math-random.* into
base, in preparation for moving the parts of conversions that don't
access HeapObjects.

Also moves uc16 and uc32 out of commons/globals.h into base/strings.h.

Bug: v8:11917
Change-Id: Ife359148bb0961a63833aff40d26331454b6afb6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2979595Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75354}