Remove our `math` function that wraps `bc`. Our math builtin is now good
enough that it can be the default implementation.
Another step in resolving #3157.
The previous change to use `argparse` for parity with every other
builtin and function introduced a regression. Invocations that start
with a negative number can fail because the negative value looks like an
invalid flag.
This makes it easy for the user to request floating point output with the
desired number of digits after the decimal point (not to be confused with
significant digits).
Note that this is just a thin wrapper so someone can say `math -s3 10 / 3`
rather than `math "scale=3; 10 /3"`.
Resolves#1643
