Make object deep-copying fallible to avoid having to implement it for the...

Make object deep-copying fallible to avoid having to implement it for the whole Stream class hierarchy
6 jobs for memoize-fetch in 9 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #204461
build

00:03:49

passed #204466
build_android

00:02:28

passed #204462
build_clang_libcpp

00:03:39

passed #204465
build_clazy

00:03:44

passed #204464
build_mingw64_fedora

00:03:34

passed #204463
build_ubuntu_16_04

00:09:56