Extern function to generate metadata (#460)

* Hack to see if #[start] works

* Hack to see if #[no_mangle] works

* Remove incrementer bin target

* Use Rust extern api and pass back InkProject struct directly

* Uglify name

* Update remaining examples to use generate metadata function

* Do not generate for ink-as-dependency to make delegator work

* Remove explicit version from abi dependency

* Remove duplication

* Remove GenerateAbi trait
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