Import md5 code from xpdf 4.02

Fixes issue 832
8 jobs for import_md5 in 12 minutes and 22 seconds (queued for 17 seconds)
Status Job ID Name Coverage
  Build
passed #759193
build

00:03:42

passed #760151
build_android

00:01:01

passed #759194
build_clang_libcpp

00:04:47

passed #759197
build_clazy

00:03:43

passed #759196
build_mingw64_fedora

00:03:50

passed #760152
build_ubuntu_16_04

00:06:58

failed #759198
build_android

00:00:19

failed #759195
build_ubuntu_16_04

00:00:34