Add comments about parsing json numbers

fix go.mod

Convert base chain priority to numeric form, if possible

To work around older nft that don't allow the symbolic forms in all
contexts.

Rename Optional() to PtrTo()

"Optional" works OK semantically as a name for Comment fields, but
seems weirder with some other fields (especially the base chain
Type/Hook/Priority fields).

update README

Fix List* errors

Replace AddRule with a Concat function

AddRule behaved weirdly with respect to comments, and didn't help when
you needed to Insert or Replace instead of Add, or if you needed to
Add with an Index or Handle. So remove it and provide a Concat method
for assembling the Rule field inside a Rule object instead.

(Also, make Concat support additional data types, so callers don't
need to do `fmt.Sprintf("%d", port)` etc themselves.)