Remove setup phase

[mannprerak2]

Closes dart-lang/native#502

• Remove the usage of _wrap functions from code.
• Remove wrapper.c and setup phase for ffigen.

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Follow up for #134.

[mannprerak2]

@dcharkes This helped remove 1 bug from #134, I think we can merge that now.

For task2 (removing setup phase) we need to have the user input the location of libclang's dynamic library. I am thinking we can have a few default locations to check, but if we can't find it there, we make the user enter the path/to/llvm/lib so that we can find the dynamic library there.

[mannprerak2]

This is complete now, I've added a new config key - llvm-lib which is required if we can't find it at the default locations.

[dcharkes]

@mannprerak2 this is marked as draft, do you want me to start reviewing?

And do you want to merge this in before releasing 2.0.0-dev.1 (edit: I see this is dev.2, so I published the previous one).

[mannprerak2]

do you want me to start reviewing?

Yes, I have added a merge commit with dart-dev branch, so that should be easier now.

[dcharkes]

Cool! Really cool to see that wrapper gone! 🥳

This shows structs by value is a nice dart:ffi feature to have! 😃