iOS setup

iOS requires some additional Rust targets for cross-compilation:

# 64 bit targets (real device & simulator):
rustup target add aarch64-apple-ios x86_64-apple-ios
# New simulator target for Xcode 12 and later
rustup target add aarch64-apple-ios-sim
# 32 bit targets (you probably don't need these):
rustup target add armv7-apple-ios i386-apple-ios