Determining the number of decimal digits in a BigInt requires examination of the ulong/uint data. It would be useful to have a straightforward method of doing so. 

I recommend making the std.internal.math.biguintcore functions peekUlong and peekUint visible at the std.bigint level by adding the following functions to std.bigint:

ulong ulongDigit(int n) pure nothrow const @safe @nogc
{
  return data.peekUlong(n);
}

uint uintDigit(int n) pure nothrow const @safe @nogc
{
  return data.peekUint(n);
}

I've readied a pull request to add these functions to std.bigint