Fix x/into perf problem when from is a small map

[imrekoszo]

Fixes #46

Running the benchmark in the issue with this patch applied:

Clojure version: 1.11.1
OpenJDK 64-Bit Server VM 17.0.3+7
aarch64 Mac OS X 12.4 10 cpu(s)

scenario \ map size	10	100	200	400	1000	100000
into with lazy sequence	Mean : 1.543115 µs Std dev : 74.096832 ns	Mean : 13.554687 µs Std dev : 212.790406 ns	Mean : 29.745477 µs Std dev : 437.020103 ns	Mean : 60.914399 µs Std dev : 760.676832 ns	Mean : 164.725324 µs Std dev : 2.798719 µs	Mean : 22.623237 ms Std dev : 1.162465 ms
into with mapping transducer	Mean : 1.078052 µs Std dev : 9.510384 ns	Mean : 8.730882 µs Std dev : 131.245677 ns	Mean : 18.825701 µs Std dev : 384.670849 ns	Mean : 42.722064 µs Std dev : 1.068641 µs	Mean : 125.213860 µs Std dev : 1.988375 µs	Mean : 14.802933 ms Std dev : 896.219721 µs
into with x/for (pairs are still allocated)	Mean : 1.156310 µs Std dev : 55.035046 ns	Mean : 9.152297 µs Std dev : 207.288913 ns	Mean : 19.185797 µs Std dev : 445.804428 ns	Mean : 38.622354 µs Std dev : 692.745875 ns	Mean : 114.455384 µs Std dev : 1.517074 µs	Mean : 14.319536 ms Std dev : 589.525520 µs
x/into with x/for (no pairs are allocated)	Mean : 887.198164 ns Std dev : 12.923610 ns	Mean : 6.467227 µs Std dev : 90.389159 ns	Mean : 13.624259 µs Std dev : 293.163150 ns	Mean : 28.783594 µs Std dev : 469.029563 ns	Mean : 94.237069 µs Std dev : 1.238456 µs	Mean : 12.222721 ms Std dev : 538.363673 µs

Clojure version: 1.10.3
OpenJDK 64-Bit Server VM 17.0.3+7
aarch64 Mac OS X 12.4 10 cpu(s)

scenario \ map size	10	100	200	400	1000	100000
into with lazy sequence	Mean : 1.644410 µs Std dev : 74.986627 ns	Mean : 13.547017 µs Std dev : 125.017175 ns	Mean : 29.833104 µs Std dev : 664.505536 ns	Mean : 60.201780 µs Std dev : 415.495164 ns	Mean : 165.078699 µs Std dev : 5.454604 µs	Mean : 20.301775 ms Std dev : 1.091515 ms
into with mapping transducer	Mean : 1.122730 µs Std dev : 8.933537 ns	Mean : 8.492994 µs Std dev : 66.904663 ns	Mean : 18.163391 µs Std dev : 437.698008 ns	Mean : 40.929538 µs Std dev : 1.055513 µs	Mean : 119.381391 µs Std dev : 1.290552 µs	Mean : 13.718284 ms Std dev : 464.498588 µs
into with x/for (pairs are still allocated)	Mean : 1.182577 µs Std dev : 22.264746 ns	Mean : 8.820678 µs Std dev : 204.735901 ns	Mean : 18.431944 µs Std dev : 393.307489 ns	Mean : 37.580225 µs Std dev : 392.065126 ns	Mean : 110.498578 µs Std dev : 676.252801 ns	Mean : 14.243484 ms Std dev : 576.655867 µs
x/into with x/for (no pairs are allocated)	Mean : 844.281597 ns Std dev : 10.933787 ns	Mean : 5.731170 µs Std dev : 70.859209 ns	Mean : 11.506273 µs Std dev : 130.308292 ns	Mean : 24.833573 µs Std dev : 328.541565 ns	Mean : 80.147593 µs Std dev : 390.259825 ns	Mean : 11.272223 ms Std dev : 735.447253 µs