enable backtraces by default and stop injecting row / col via a macro

[thestinger]

The row and column aren't usually useful, because there's no context. An error pointing to unwrap is almost completely useless. On platforms where backtraces are already working, it should replace the current failure messages. Failure indicates a logic error (bug) and context is important for tracking it down.

An abort would be far more usable because it provides a whole core dump including a backtrace with all of the local variables.

Relocations from the bloated failure messages are the cause of over 50% of the time taken by rustc for hello world on Windows. It's much less ridiculous on other platforms, but the bloat is still a very serious performance issue in many performance critical parts of the standard libraries and in user code.

[steveklabnik]

A change of this magnitude, these days, requires an RFC. We have two competing ones open right now: rust-lang/rfcs#2091 and rust-lang/rfcs#2154

I'm going to close this in favor of those.