Adds the concept of a "temporary working area" to flat_tree. Nodes in this area belong to a partially parsed message, and are hidden from the user. Now flat_tree can be used as the receive response without explicitly handling partial messages.
Changes flat_tree::get_view return type from const vector& to span.
Adds flat_tree::capacity.
Splits generic_flat_response tests to a separate file and adds extra cases.
close#369
* Makes flat_tree implementation use a custom buffer. This allows:
* Never dangling nodes (previously, node values could dangle after calling reserve() or if notify_done() wasn't called).
* Reduced memory consumption
* Increased runtime speed
* Changes flat_tree assignment to the usual signature and semantics
* Fixes a bug causing an assertion to trigger when copy-constructing an empty flat_tree.
* Changes basic_node operator== and operator!= return type
* Adds generic_flat_response, basic_tree, tree, view_tree, flat_tree to the reference page.
* Adds a missing resp3:: qualifier to all names in the reference page that belong to the resp3 namespace.
* Adds reference documentation to flat_tree.
* Mentions generic_flat_response in the discussion.
* Adds operator!= for basic_node to basic_node's reference page.
* Adds test_flat_tree.
close#357close#354close#352
