Add std to builtins

I created a new tests/test_stats.py file instead of using tests/test_stat.py because the latter file is being used for benchmarking, not unit testing. The latter file includes a `stdev(x)` function, but only works for a single axis and does not have any unit tests for it.

This PR also adds `Assert.float_close,` a helper method for testing the relative equality of two floating point numbers.