`getDigit` Was Added To `std.bigint`

$(REF getDigit, std, algorithm, BigInt) gives the `ulongs` or `uints`
that make up the underlying representation of the `BigInt`.

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import std.bigint;

auto a = BigInt("1000");
assert(a.getDigit(0) == 1000);

auto b = BigInt("2_000_000_000_000_000_000_000_000_000");
assert(b.getDigit(0) == 4584946418820579328);
assert(b.getDigit(1) == 108420217);
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