Make CommonBigInteger/Decimal public

[MisterAssm]

kt-math 0.7.0 cannot be a pure porting of java.math.* BigInteger and BigDecimal, representative classes, are marked as internal, the constructors are then not available to users.

So for example, the sign-magnitude representation of a BigInteger in a BigInteger cannot be called for the creation of a BigInteger (for an RSA Kotlin/JS implementation)

[gciatto]

@MisterAssm I should have added for factories on the interfaces to address your issue.
Is that ok for you?