Rust 1.51 allows addressesing a long-standing TODO which was that we
shouldn't need to build a vendored copy of openssl on most sensible unix
systems.
We do require a vendored copy on macOS and Windows, but due to the way
that Cargo's feature resolver worked, it wasn't possible for this
requirement to be respected.
Rust 1.51 introduces `resolver="2"` which can deal with this feature
resolution!
https://doc.rust-lang.org/nightly/cargo/reference/features.html#feature-resolver-version-2
The upshot of this is that building wezterm on real unix systems that
are not macos will now link against the system libssl, resulting in both
a shorter compile time and less headaches arising from having a slightly
different openssl used by wezterm than the rest of the system.
cc: @jsgf
