pw_string: add pw::string::Copy methods

Adds pw::string::Copy helper methods as a safer alternative to
std::strncpy which unfortunately does not always null terminate.

In addition, the existing StringCopy methods are renamed to
StringOrNullCopy to denote that they support nullptr source strings
unlike pw::string::Copy.

Change-Id: I046c12da02721c5ad2f6601b9ac742d9dfa90c71
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/42931
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>