iterator: add a bit of documentation
auto merge of #5955 : thestinger/rust/iterator, r=graydon
librustc: Remove `fail_unless!`
librustc: Implement fast-ffi and use it in various places
rustllvm: Fix RustWrapper.cpp
librustc: Switch the `@`s in types with `~`
librustc: Take primitive types out of the type hash table.
librustc: Remove def_ids from types.
librustc: Optimize metadata::decoder::item_name.
libstd: Micro-optimize vuint_at
rt: Fix scalability problem with big stacks on 32 bit
librustc: Improve inlining behavior.
llvm: Fixes for RustWrapper.
test: Rewrite nbody and spectralnorm shootout benchmarks
test: Add fannkuch-redux and fasta-redux shootout benchmarks
test: Add k-nucleotide
Move shootout-k-nucleotide to bench
test: Implement pidigits and reverse-complement
librustc: WIP patch for using the return value.
rt: Remove dump_stacks