Implement Swedish/Cash rounding fixes #63

[SchumacherFM]

Hey,

I need the following guidance: Have I implemented the rounding correctly? Are there any architecture or other bugs? Is the panic correct? Are comments correct and consistent? How can I (or you) reduce the allocations? There seems to be some tricks in the library to avoid allocations but I'm not familiar with the math/big package. You can also work on the allocs to reduce them. Let me know.

RoundCash aka Cash/Penny/öre rounding rounds decimal to a specific
interval. The amount payable for a cash transaction is rounded to the nearest
multiple of the minimum currency unit available. The following intervals are
available: 5, 10, 15, 25, 50 and 100; any other number throws a panic.

    5:   5 cent rounding 3.43 => 3.45
   10:  10 cent rounding 3.45 => 3.50 (5 gets rounded up)
   15:  10 cent rounding 3.45 => 3.40 (5 gets rounded down)
   25:  25 cent rounding 3.41 => 3.50
   50:  50 cent rounding 3.75 => 4.00
  100: 100 cent rounding 3.50 => 4.00

For more details: https://en.wikipedia.org/wiki/Cash_rounding

BenchmarkDecimal_RoundCash/five-4         	 1000000	      1918 ns/op	    1164 B/op	      30 allocs/op
BenchmarkDecimal_RoundCash/fifteen-4      	  300000	      4331 ns/op	    2940 B/op	      74 allocs/op

[victorquinn]

This looks excellent @SchumacherFM ! Thanks for the addition and for the thorough test coverage :)

[SchumacherFM]

Thanks for merging!