Cargo mode for building rust contracts?

[blckngm]

trafficstars

Add an option to use cargo instead of cross?

This would require natively installed riscv64-unknown-elf-gcc instead of cross and docker.

This can be an option in the config file and can be overridden with an environment variable.

[blckngm]

Just noticed that gcc is not necessary if there's no C code to build. Default linker is rust-lld.