make release builds the default

This cuts the compilation time for a trivial Rust program by 15-20% on
Windows due to a massive improvement in `rustc` start-up time. The debug
logging has a very high cost because it makes extensive use of mutable
global variables and those result in expensive relocations.

Creating debug builds by default discourages the usage of debug
assertions. The few debug assertions not removed before landing a pull
request are a significant performance problem for types like `RefCell`.

The defaults should cater to users or packagers building Rust rather
than compiler developers. A compiler developer can be expected to
override a default flag, but the same cannot be said of someone who
lacks the same in-depth knowledge of the project.

Even someone working on the standard libraries is not going to want to
pay the high cost for debug logging. Many people who contribute to the
compiler don't use the feature either, because a debugger tends to work
a lot better than inconsistent / bit-rotted logging code.