Allow use of automake 1.13
Move implementation of #to_ffitype for interfaces
Make ITypeInfo#to_typeinfo return type FFI understands

This simplifies #layout_specification_type.
Include SafeConstantName in InfoExt::IRegisteredTypeInfo

This makes it clearer what extensions are present on IRegisteredTypeInfo.
Merge #layout_specification_type into #to_ffitype

There are no cases where they need to return different results, although
some results will only occur during layout specification or function
definition respectively.