auto merge of #15068 : erickt/rust/mem-inline, r=pcwalton

This is a couple micro-optimizations I've been sitting on for a while. This inlines a couple functions that are important to the `std::io::mem`. Ultimately, this results in about a 15% performance increase in some micro-benchmarks for my [libserialize](https://github.com/erickt/rust-serde) rewrite.