Replace the String values with Cow<'static, str> which lets us hold references
to static strings without copying them onto the heap. Since most added values
are static, this should save on heap usage and runtime cost.
These are intertwined: older versions of Rust don't support the newer
NDK, but the newer Rust can't successfully compile BoringSSL against
the older NDK.
This requires a boring-sys update to find the Android NDK sysroot in
the right place.
Signal has a fork of curve25519-dalek to add some features that are
used by zkgroup. However, libsignal-protocol and poksho don't use
those features directly, and thus they don't depend on our fork
specifically. Anyone outside of Signal using libsignal-protocol can
thus use the standard curve25519-dalek and avoid building it twice.
Signal will continue using our fork thanks to the workspace patch in
the root Cargo.toml.
Additionally, remove all the passthrough features for customizing
curve25519-dalek; we don't use any of them, and clients can always
specify them directly.
