optimize query operations

[OmarTawfik]

Examples:

• go_to_nth_child might be quicker to start from the end (like go_to_last_child) - actually a Cursor optimisation.
• the combinators are strictly FIFO during the search, so can be allocated and deallocated using a single block of memory, avoiding all allocation.
• binding names can be shared, rather than allocated.
• before searching over a tree, a hash map can be built keyed by the FIRST SET of the kinds of each query, avoiding the cost of iterating over the queries for each node.

Please check the TODO(#871) comments added in #873.