rename `TaggedLen` to `LocatedLength`

[alejandro-vaz]

as part of using more descriptive names in #639, TaggedLen should be renamed to something like LocatedLength or similar

name suggestions are welcome

update every use of it

[bolshoytoster]

I think I actually prefer TaggedLen/TaggedLength. It says what it is (a tag and a length).

[alejandro-vaz]

yeah but a "tag" can be anythin