kos-mobile BigNumber module (#100)

* feat: comparison methods, pow, absolute and test cases

* chore: update dependencies

* chore: adjust deny checks

* feat: added support for rational numbers

* chore: avoid errors return on comparison methods

* chore: adjust fmt

* chore: enhance number validation & accept exponential input

* chore: use BigDecimal insead of f64 for checking valid number

* chore: changed to new BigNumber struct

* chore: enhanced BigNumber struct fields