Fix cross compilation issue

[alphaville]

This is work in progress

Main Changes

Firstly, each cargo package has its own .cargo/config.toml file with the following contents:

[target.x86_64-apple-darwin]
rustflags = [
  "-C", "link-arg=-undefined",
  "-C", "link-arg=dynamic_lookup",
]
[target.aarch64-apple-darwin]
rustflags = [
  "-C", "link-arg=-undefined",
  "-C", "link-arg=dynamic_lookup",
]

# This is for cross-compiling to ARM architecture (e.g., Raspberry Pi)
[target.arm-unknown-linux-gnueabihf]
linker="arm-linux-gnueabihf-gcc"

Then we can cross-compile with cargo build --target=arm-unknown-linux-gnueabihf.

Secondly, we now cross-compile for RPi on GitHub Actions (see test/test_raspberry_pi.py and ci.yml).

Lastly, I tested this on a Raspberry Pi and it works.

Other changes:

• Updated website docs here
• Updated min cmake version from 2.8 to 3.5
• Updated auto-generated example C/C++ bindings

Associated Issues

• Closes Compilation for embedded target #366
• Opens Cross-compilation with Python bindings: PYO3_CROSS_LIB_DIR not set #369

We're still getting an error when cross-compiling with .with_build_python_bindings(), but we'll address it in a separate PR (see issue #369).

TODOs

• Documentation
• All tests must pass
• Update CHANGELOG(s)
• Update webpage documentation!
• Bump versions (in CHANGELOG, Cargo.toml and VERSION)

[ruairimoran]

looks good, some comments