Allow customizing backtrace printing beyond the feature flag

[jrose-signal]

This adds a builder Config for customizing the panic hook, which currently only has one option, backtrace_mode. There are three modes:

• Off: no backtrace
• Unresolved: addresses only
• Resolved: addresses and symbols if possible (default)

The builder is implemented such that if symbol resolution is not requested, that part of the 'backtrace' crate can be eliminated as dead code if the client is compiled with LTO (thin or full).

---

Fixes #10. Tested with

• cargo +1.45 test
• cargo +1.45 test --all-features
• cargo +1.45 check --all-targets
• cargo +1.45 check --all-targets --all-features
• cargo +1.45 doc (a little suboptimal, [`foo`] syntax is newer than that)
• cargo +1.45 doc --all-features
• RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features

as well as compiling signalapp/libsignal against this branch of log-panics and disabling symbol resolution.

[sfackler]

LGTM other than the one comment.