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.
This `start` should be a symmetry of the `start` when `axis == 0`. 

The logic of thest two parts is the same. I guess it is possible to combine these two parts (`axis == 0` and `axis == 1`) without duplicating the code (by introducing extra variables to record the index). You can check if it is doable.