`deny(unsafe_op_in_unsafe_fn)` in backtrace-rs

[workingjubilee]

Because backtrace-rs is included as a submodule and has platform-specific code, this lint is currently allowed inside std. I intend to remove this allow in std for this crate, and deny this lint inside backtrace.

There is platform-specific code in this crate. For most platforms, most of it is fairly limited, and limited to safe functions, so that there is no concern with this lint. There is, however, the divide between DWARF (or "Unix") and PDB (or "Windows"), on which pretty much everything else about this crate hinges. This should be... comparatively easy? to fix?