rustc: Check for null before dropping ~objects. #5192
auto merge of #6705 : brson/rust/issue-5192, r=catamorphism

r? @catamorphism 

This could probably refactored to more closely mirror the code for `@objects`, but I'm not inclined to do so.
core: remove iter_bytes helper functions
cleanup warnings from libstd
cleanup warnings from libextra
cleanup warnings from libsyntax
cleanup warnings from librustc
cleanup warnings from librusti
cleanup warnings from librustdoc
cleanup warnings from compiletest
cleanup warnings from librustpkg
Fix some std/extra language in the tutorial
auto merge of #6690 : erickt/rust/cleanup-warnings, r=brson

Simple patch series to fix up all the warnings a rustc compile is giving at the moment. It also fixes a NOTE in `to_bytes.rs` to remove the `to_bytes::iter_bytes_<N>` functions.
Change how the input from rusti is formatted to allow duplicate definitions
Change the generic 'print' function to 'pp' for rusti