feat: add binary_trees_gc benchmark that explicitly invokes System.gc

Add a new benchmark variant of the binary_trees test that calls System.gc() after
each tree construction phase to measure garbage collection overhead. The benchmark
creates a stretch tree, a long-lived tree, and iterates through depths 4 to 12
with explicit GC calls between iterations. Register the new benchmark in
util/benchmark.py with expected output values matching the original binary_trees
benchmark.

util/benchmark.py

@@ -59,6 +59,14 @@ BENCHMARK("binary_trees", """stretch tree of depth 13 check: -1
 32 trees of depth 12 check: -32
 long lived tree of depth 12 check: -1""")
 
+BENCHMARK("binary_trees_gc", """stretch tree of depth 13 check: -1
+8192 trees of depth 4 check: -8192
+2048 trees of depth 6 check: -2048
+512 trees of depth 8 check: -512
+128 trees of depth 10 check: -128
+32 trees of depth 12 check: -32
+long lived tree of depth 12 check: -1""")
+
 BENCHMARK("delta_blue", "14065400")
 
 BENCHMARK("fib", r"""317811