Add a g_ref_string_new_len() to allow creating from non-NUL byte arrays

A lot of GLib APIs provide a string length and explicitly say that the strings
are not NUL terminated. For instance, parsing XML using GMarkupParser or
reading packed binary strings from mmapped data files.
8 jobs for wip/hughsie/g_ref_string_new_len in 9 minutes and 28 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #59741
cross-android_api21_arm64

00:02:34

passed #59742
cross-android_api28_arm64

00:02:47

passed #59743
cross-mingw64

00:06:05

passed #59740
fedora-x86_64

00:04:58

canceled #59746
freebsd-11 allowed to fail
freebsd-11-x86_64

passed #59744
win32
msys2-mingw32

00:06:43

passed #59745
win32
vs2017-x64

00:05:50

 
  Coverage
passed #59747
coverage

00:02:44

69.9%