Simplified API for custom types (yet again)

[AjayBrahmakshatriya]

This change further simplifies custom types in 2 ways -

1. Custom types are no longer required to inherit from builder::custom_type, although they can choose to do so for appending template args to names. Custom types are also not required to define the type_name member. If they don't, an automatically generated name will be assigned. This can be used in combination with c_code_generator::generate_struct_decl to create custom types.
2. Members are no longer required to use member_of, although they can use it if members are created outside the constructor itself or if they are using specialization for dyn_var. Instead they can simply with with_name to name the members or leave the member uninitialized in which case it is assigned a generated name. This can be again used in combination with c_code_generator::generate_struct_decl to create custom types.

Sample56 has been added to test this. Older APIs are still all supported completely.