Added bench.py and bench_runner.c for benchmarking

These are really just different flavors of test.py and test_runner.c
without support for power-loss testing, but with support for measuring
the cumulative number of bytes read, programmed, and erased.

Note that the existing define parameterization should work perfectly
fine for running benchmarks across various dimensions:

./scripts/bench.py \
    runners/bench_runner \
    bench_file_read \
    -gnor \
    -DSIZE='range(0,131072,1024)'

Also added a couple basic benchmarks as a starting point.

scripts/summary.py

@@ -28,7 +28,8 @@ MERGES = {
     'add': (
         ['code_size', 'data_size', 'stack_frame', 'struct_size',
             'coverage_lines', 'coverage_branches',
-            'test_passed'],
+            'test_passed',
+            'bench_read', 'bench_prog', 'bench_erased'],
         lambda xs: sum(xs[1:], start=xs[0])
     ),
     'mul': (