Should `backtrace` dependency be optional?

[vi]

Although nominally optional, backtrace is effectively required if you want to use Error or bail!.

This hinders portability, as backtrace depends on C code:

cargo:warning=src/libbacktrace/macho.c:50:2: error: #error Attempting to build Mach-O support on incorrect platform

Shall failure have some feature combination to build projects depending on failure without backtrace support, yet with typical failure's convenience features?

[daboross]

See also #127