odu: A generic filesystem benchmarking utility for fuchsia

One of the things odu is used for is to run the following test cases:

• sequential/write/8192: Measures the time to write blocks of size 8KiB in turn to a newly-created file, using the system filesystem, without any explicit syncing or flushing. The resulting metric is the time taken for each per-block write to return.
• sequential/write/random_size: Measures the time to write blocks of random size less than 8KiB in turn to a newly-created file, using the system filesystem, without any explicit syncing or flushing. The resulting metric is the time taken for each per-block write to return.
• sequential/read/8192: Measures the time to read blocks of size 8KiB in turn from a file, using the system filesystem. The resulting metrics is the time taken for each per-block read to return.
• sequential/read/random_size: Measures the time to read blocks of random size less than 8KiB in turn from a file, using the system filesystem. The resulting metric is the time taken for each per-block read to return.