auto merge of #5278 : brson/rust/logplusplus, r=brson

auto merge of #5278 : brson/rust/logplusplus, r=brson

r?

`log` can  polymorphically log anything, but debug!, etc. requires a format string. With this patch you can equivalently write `debug!(foo)` or `debug!("%?", foo)`.

I'm doing this because I was trying to remove `log` (replacing it with nothing, at least temporarily), but there are a number of logging statements that just want to print an arbitrary value and don't care about the format string.

I'm not entirely convinced this is a good change, since it overloads the implementation of these macros and makes their usage slightly more nuanced.