auto merge of #5293 : brson/rust/logging, r=brson

r? @graydon

This removes `log` from the language. Because we can't quite implement it as a syntax extension (probably need globals at the least) it simply renames the keyword to `__log` and hides it behind macros.

After this the only way to log is with `debug!`, `info!`, etc. I figure that if there is demand for `log!` we can add it back later.

I am not sure that we ever agreed on this course of action, though I *think* there is consensus that `log` shouldn't be a statement.

diff --cc src/librustc/back/link.rs
Simple merge

diff --cc src/librustc/middle/trans/build.rs
Simple merge

diff --cc src/libstd/getopts.rs
Simple merge

diff --cc src/libsyntax/parse/parser.rs
Simple merge