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_crate.cpp

@@ -41,8 +41,8 @@ rust_crate::mem_area::mem_area(rust_dom *dom, uintptr_t pos, size_t sz)
     base(pos),
     lim(pos + sz)
 {
-  DLOG(dom, rust_log::MEM, "new mem_area [0x%" PRIxPTR ",0x%" PRIxPTR "]",
-           base, lim);
+  DLOG(dom, mem, "new mem_area [0x%" PRIxPTR ",0x%" PRIxPTR "]",
+       base, lim);
 }
 
 rust_crate::mem_area