Add deriving_eq to Cell.
auto merge of #5318 : jdm/rust/deriving_cell, r=pcwalton

r? @pcwalton
auto merge of #5305 : brson/rust/mut_buf_as_slice, r=brson

r?

Like `buf_as_slice` but for mutable pointers and slices.
Remove uses of log
core: Remove logging constants
Remove the log keyword (by renaming it to __log)

We can't quite remove logging from the language, but this hides the
keyword.