tutorial: use "owned box" consistently
Stop building clang

Removing it from the tree is an ordeal and there is no official way
to disable clang via LLVM's Makefiles so this edits the Makefile
in llvm/tools after running configure.
Register snapshots
auto merge of #5615 : brson/rust/noclang, r=brson

r? @graydon

Closes #5403
auto merge of #5623 : brson/rust/snapshot, r=brson
tutorial: improve the owned boxes section
tutorial: improve the managed boxes section
tutorial: add an example of freezing a managed box
auto merge of #5624 : thestinger/rust/tutorial, r=pcwalton
Merge remote-tracking branch 'remotes/origin/incoming' into serial