#61

#62

4.x branch - Fix alpm-sys build.rs version check

Fix UB when using empty SigList

Fix callback event type to enum variant mapping

Encapsulate return type with `Option` enum instead of panicking

Update libalpm version in README.md

Remove deprecated SPDX License ID from Cargo.toml

minor change: http => https

Fix API breakage relating to lifetime tweaks and bitflags attributes