Ultimathnum 0.9.0

Shiny new things and better old things.

• Various API and non-API changes (0.y.z)
• #84 Improve: arbitrary integer multiplication
• #83 Add: BinaryInteger/factorial()
• #82 A more number-y Signum
• #81 Improve: TextInt
• #80 Add: BinaryInteger/negate()
• #79 Hoist GCD, XGCD, and remove unsigned constraint
• #76 Add: BinaryInteger/isqrt()
• #75 Generic two-way comparisons
• #74 Add: Interoperable
• #73 Using Randomness in Stdlib methods
• #72 Generic exponent in BinaryInteger/power(_:)
• #71 Free multiplication in BinaryInteger/power(_:)
• #70 Add: Division/exactly()
• #66 Miscellaneous 0.9.0 stuff
• #64 Divider21<T>
• #63 Division by constant in TextInt
• #62 Protocol: Guarantee