turnip: Add emit functions in a header.

turnip: Add emit functions in a header.

This adds a radv-style check_space functions + emit functions.

Also puts them in a header as a bunch of inlines, so

(1) we can use them from meta code.
(2) they are inline for performance as these are common and small.

Did not put them in tu_private.h as a bunch of inlines only
clutters up that huge headerfile.

Precise error propagation for memory allocation failures is still
todo.