Pimplify abigail::ir::function_type

Pimplify abigail::ir::function_type

	* include/abg-ir.h (function_type::priv_): Declare new data
	member.
	(function_type::<all the methods>): Move the inline methods out of
	line in src/abg-ir.cc.
	(function_type::{return_type_, parms_}): Move these ...
	* src/abg-ir.cc (function_type::priv::{return_type_, parms_}):
	... Here.
	(struct function_type::priv): New type for the private data of
	function_type.
	(function_type::<all the methods>): Move the previously inline
	methods of function_type here.  Adjust them to tap into priv_->*
	to get the private data members.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>