make nn non-allocating by having it "track" scalars

[KristofferC]

Fixes #227

From the benchmark in that issue:

# Before
julia> @btime nn($tree, $p)
  52.347 ns (4 allocations: 128 bytes)
(19, 0.14043712730996852)

# After
julia> @btime nn($tree, $p)
  29.732 ns (0 allocations: 0 bytes)
(19, 0.14043712730996852)