Overhaul logging system in runtime

See https://github.com/graydon/rust/wiki/Logging-vision

The runtime logging categories are now treated in the same way as
modules in compiled code. Each domain now has a log_lvl that can be
used to restrict the logging from that domain (will be used to allow
logging to be restricted to a single domain).

Features dropped (can be brought back to life if there is interest):
  - Logger indentation
  - Multiple categories per log statement
  - I possibly broke some of the color code -- it confuses me

src/rt/rust_task.h

@@ -82,8 +82,6 @@ rust_task : public maybe_proxy<rust_task>,
     void check_active() { I(dom, dom->curr_task == this); }
     void check_suspended() { I(dom, dom->curr_task != this); }
 
-    void log(uint32_t type_bits, char const *fmt, ...);
-
     // Print a backtrace, if the "bt" logging option is on.
     void backtrace();