Use apd primitives for faster comparisons

[bcremer]

before:

$ go test -bench='BenchmarkAmount_Is' -benchmem
goos: darwin
goarch: arm64
pkg: github.com/bojanz/currency
cpu: Apple M2 Max
BenchmarkAmount_IsPositive-12           187741496                6.244 ns/op           0 B/op          0 allocs/op
BenchmarkAmount_IsNegative-12           191502921                6.224 ns/op           0 B/op          0 allocs/op
BenchmarkAmount_IsZero-12               202620387                5.994 ns/op           0 B/op          0 allocs/op
PASS
ok      github.com/bojanz/currency      5.999s

after:

$ go test -bench='BenchmarkAmount_Is' -benchmem
goos: darwin
goarch: arm64
pkg: github.com/bojanz/currency
cpu: Apple M2 Max
BenchmarkAmount_IsPositive-12           620603521                1.753 ns/op           0 B/op          0 allocs/op
BenchmarkAmount_IsNegative-12           826252094                1.447 ns/op           0 B/op          0 allocs/op
BenchmarkAmount_IsZero-12               916716909                1.332 ns/op           0 B/op          0 allocs/op
PASS
ok      github.com/bojanz/currency      4.318s

[bojanz]

Great work, thank you!