SkStream: remove some WStream functions from public api

move functions to SkStringPriv.h

also add SkStrAppendU32Hex() function, and re-write
SkString::insertHex() to use SkStrAppendU32Hex.

add unit tests.

Change-Id: Ieda98fb4106db71565b607e593713a91a5ddd892
Reviewed-on: https://skia-review.googlesource.com/151986
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
15 files changed