Multiple implementations

[plicease]

There could be some advantages to supporting multiple implementations down the road. Examples:

• Win32::API may provide better performance or features on Windows platforms
• GNU ffcall may support some platforms that libffi does not (skeptical)
• Alternate implementations of Perl, such as p2, gperl or moe or even Perl 6

I had this in mind when I was designing the interface, but I haven't been implementing it in a particularly pluggable way unfortunately. Tasks include:

• Moving the libffi specific Perl code into FFI::Platypus::Impl::Libffi
• FFI::Platypus becomes a subclass of FFI::Platypus::Impl::...
• split the XS and C code into libffi specific stuff and non-libffi specific stuff. For example ffi_platypus_call.h is decidedly libffi specific. The record code should not be.
• Remove the libffi objects from the structs in ffi_platypus.h. Right now we are using the size and type fields for logic. This is probably the most significant task.