Move FFI::DataConverter to Ruby

[eregon]

• Include DataConverter in StructByReference in Ruby code to simplify ordering.

See #660 for context.

[eregon]

This is now ready for review.

[larskanis]

LGTM

[headius]

Ruby impl seems to have most of the same behavior as the C code, so I suspect this should be fine.

[eregon]

Can we merge this?

@tduehr Could you review?

I think this doesn't touch performance critical-code, but I'm happy to run some benchmarks if you tell me which ones would be relevant for this.

The behavior seems similar to the DataConverter class in JRuby, except that one has the extra reference_required and reference_required? methods.

[tduehr]

There's more to it than just the module itself. I'll take a look at usage today.

I expect this code is used in converting from ruby to C/JAva and back... Any benchmark covering that should be good.

Can you get the benchmark suite working, or at least find the holes? That'd be awesome too...

[headius]

@tduehr Even if this is on the hot path it's probably a lateral move changing this code to Ruby. It may even be faster in some cases since the C code (and the Java code, for that matter) does uncached instance variable lookups.

[tduehr]

The licensing is all over the place. We'll want to consolidate that at some point.

[eregon]

I added the license header.

[eregon]

For the record, I checked, and none of the benchmarks, nor the tests ever calls FFI::DataConverter#to_native or FFI::DataConverter#from_native, and DataConverter#native_type is only called during require 'ffi'.

It's easy to adapt bench_chmod.rb to use it by changing :string arguments to :strptr (which is not useful in practice, :strptr is only useful as a return type, but then it overrides from_native which doesn't call in DataConvert anymore), and as expected the performance difference is insignificant.