Avoid quadratic complexity when coalescing text nodes in html in golang.org/x/net

When coalescing adjacent text nodes in addText and fosterParent, appending to an existing TextNode's Data via string concatenation (n.Data += text) reallocated and copied the accumulated string on every token. Inputs containing many short text runs separated by ignored end tags (such as "a") caused O(n^2) allocations and byte copies.

Accumulate coalesced text in per-node byte slices during parsing and materialize Data strings once at the end of parse.