log: Logger receiveis a LogRecord

The logging macros now create a LogRecord, and pass that to the
Logger, instead of passing a `level` and `args`. The new signature is:

    trait Logger {
        fn log(&mut self, record: &LogRecord);
    }

The LogRecord includes additional values that may be useful to custom
loggers, and also allows for further expansion if not values are found
useful.

DefaultLogger's formatting was taken from Python's default formatting:
`LEVEL:from: message`

Also included: fmt::Arguments now implement Show, so they can be used to
extend format strings.

[breaking-change]