Remove the __log function for __log_level

Also redefine all of the standard logging macros to use more rust code instead
of custom LLVM translation code. This makes them a bit easier to understand, but
also more flexibile for future types of logging.

Additionally, this commit removes the LogType language item in preparation for
changing how logging is performed.

src/libstd/std.rs

@@ -218,6 +218,7 @@ mod std {
     pub use option;
     pub use kinds;
     pub use local_data;
+    pub use logging;
     pub use sys;
     pub use unstable;
     pub use str;