support for generics in compile based IDL generation

[kklas]

The compile IDL generation method (#2011) doesn't work for types with generics.

The code it generates is similar to this:

impl SomeAnchorSerializeType {
   <... idl gen helper methods ...>
}

When the type has generics it will generate this:

impl<T> SomeAnchorSerializeType<T: AnchorSerializeType> {
   <... idl gen helper methods ...>
}

Which is fine, but we can't call these methods because we'd have to specify which exact type T to call them for which we don't know.
And in practice knowing the type T shouldn't be necessary because the "idl gen helper methods" don't rely on T. It's just that rust won't allow us to call them without specifying T.

But we can go around this by generating an empty placeholder type and then do impl on it instead of the original type:

pub struct __anchor_private_idl_gen_SomeAnchorSerializeType {}

impl __anchor_private_idl_gen_SomeAnchorSerializeType {
   <... idl gen helper methods ...>
}

Then we call the methods on the placeholder type instead of the original one.

[kklas]

Solved in #2011