erlcapnp cheats when encoding lists of text fields

[bucko909]

ast_encode_ptr(N, PtrLen0, #ptr_type{type=list, extra={text, TextType}}, VarName, Line) ->
    % This is a bit of a hack; we encode a list of text fields as a list of structs which have the first field being text.
    % They should really be anyPointer, I think, which doesn't need a header tag word.

This instructs to use ast_encode_struct_list_/3 to encode the text list.

Should really have an ast_encode_ptr_list_/1 which takes EncodeFun only.

That function should be functionally identical except that:

• It should not add a tag word.
• It should assume StructLen is 1 (each element of the list is just a single pointer).

There should also be a test to make sure this works.