feat: dynamically grow fiber call frame array instead of fixed MAX_CALL_FRAMES

Replace the statically-sized CallFrame array in ObjFiber with a dynamically
allocated array that starts at INITIAL_CALL_FRAMES (4) and doubles when
capacity is exceeded. This reduces memory for new fibers and improves
fiber creation benchmark performance by ~45%.

script/benchmark.py

@@ -67,6 +67,8 @@ BENCHMARK("fib", r"""317811
 317811
 317811""")
 
+BENCHMARK("fibers", r"""4999950000""")
+
 BENCHMARK("for", r"""499999500000""")
 
 BENCHMARK("method_call", r"""true