Fixed harlmess duplicate generated code with Vec<T> and possibly other cases.

[stephan57160]

As discussed on #435, the code generation may create duplicate generated code. Most of the time, it's harmless, but it may bring troubles later or with other parts of code.

This happens in the following scenario when there is a function having a Vec in parameter and another one as return value:

    class Foo {
        fn Foo::return_is_array_u8() -> Vec<u8>;
        fn Foo::param_is_array_u8(_: Vec<u8>);
    }

If there is :

• only one of them,
• or a unique function having both, the duplicate generated code is not seen.

In this fix, a foreign_code_cache is added to CppContext. This cache is checked before writing from cpp-include.rs code rules. When the code entry is already present in the cache, it's no more written to FileWriteCache.

[stephan57160]

Then, I don't see yet why there is a test case failure ...

[Dushistov]

Thanks for PR, I don't like cloning on every lookup, but I suppose this is can be addressed later.

[stephan57160]

I don't like it either, but did not find anything better, for now... Thx to accept it anyway.