Improve generated interface impls

[headius]

We should have a look at how we are generating the implementations of users' interfaces and see if we can make improvements, based on newer ASM and JDK features:

• Reduce the cost of generating and loading the code.
• Improve the performance of dispatch through the generated code.

This code was originally written by Wayne Meissner many years ago, so there should be some good improvements possible.

Note that #249 was discovered because JRuby's faster-starting "dev mode" disables the ASM-based code generation in jnr-ffi, which does have a measurable impact on startup time.

[headius]

Mostly this needs an audit to see what could be improved, as the code is very old.

I added this as a placeholder because we were debating whether to turn the generation off for the --dev flag, which does have a small but measurable impact on startup. Ideally we would not ever turn it off because the non-generated path uses java.lang.Proxy which has perf and behavior problems.