feat: add benchmark runner with fib implementations and OS.clock primitive

Add a benchmark runner script (benchmark/run_all) that executes multiple
fibonacci implementations across languages (Lua, Python, Ruby, Wren) and
computes mean, median, and standard deviation from 7 runs. Update existing
fib benchmarks to loop 5 times at fib(30) with elapsed timing. Introduce
OS.clock primitive in C core and OS class to support timing in Wren.
Adjust number formatting to "%.14g" for consistent output precision.

benchmark/fib.py

@@ -1,5 +1,10 @@
+import time
+
 def fib(n):
   if n < 2: return n
   return fib(n - 1) + fib(n - 2)
 
-print fib(35)
+start = time.clock()
+for i in range(0, 5):
+  print(fib(30))
+print("elapsed: " + str(time.clock() - start))