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%.

test/benchmark/fibers.wren

@@ -0,0 +1,16 @@
+// Creates 10000 fibers. Each one calls the next in a chain until the last.
+var fibers = []
+var sum = 0
+
+var start = IO.clock
+
+for (i in 0...100000) {
+  fibers.add(new Fiber {
+    sum = sum + i
+    if (i < 99999) fibers[i + 1].call()
+  })
+}
+
+fibers[0].call()
+IO.print(sum)
+IO.print("elapsed: ", IO.clock - start)