Dividing BitLen by math.Log2(10) is what math/big does underneath
Not including the Int64/Uint64 check makes this slightly slower than old method
Included 2 benchmarks, for 10 digit numbers & 100 digit numbers:
-- before
> go test -bench=NumDigit -run=NumDigit
goos: linux
goarch: amd64
pkg: github.com/shopspring/decimal
cpu: AMD Ryzen 7 7840U w/ Radeon 780M Graphics
BenchmarkDecimal_NumDigits10-16 18317293 63.87 ns/op
BenchmarkDecimal_NumDigits100-16 3645015 329.6 ns/op
-- after
...
BenchmarkDecimal_NumDigits10-16 143781325 8.488 ns/op
BenchmarkDecimal_NumDigits100-16 5931247 207.4 ns/op
