Changed configuration option `decimals` to `precision`
Some internal refactoring
Implemented BigNumber support for functions `exp`, `log`, and `log10`.
Implemented `BigNumber` support for function `diag`
Removed errors for stuff deprecated since v0.16.0
Added unit alias `lbs` (pound mass).
Implemented DimensionError and IndexError
Moved ArgumentsError from lib/type/ to lib/util/