- There is no need to materialize all the content of a given Node when we can
simply compute its length directly, saving a lot of memory, on the order of
several megabytes on my instance, with peaks at a couple of dozen.
- One might object to the usage of a recursive construct, but this is a direct
port of goquery's Text method, so this change doesn't make anything worse.
- The computation of linkLength can be similarly computed, but this can go in
another commit, as it's a bit trickier, since we need to get the length of
every Node that has a `a` Node as parent, without iterating on the whole
parent chain every time.
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