NativeCalculator::mul() can now handle multiplication natively in mor…

…e cases

Add precision about toFloat() and Infinity

Prepare for release 0.8.0

New method: hasNonZeroFractionalPart()

Use nullable type for Calculator::set()

Export-ignore dev files

Remove deprecated class & methods

Deprecate ArithmeticException, renamed to MathException

Strict comparisons

CI on PHP 7.2, add HHVM notes