std.bigint.BigInt: special case x & non-negative int to avoid unnecessary allocation

[n8sh]

x & 1 is a natural way to check the low bit of std.bigint.BigInt x but that allocates unlike the less natural x.getDigit!uint(0) & 1. Examples of this appearing in Phobos are std.bigint.powmod(BigInt, BigInt, BigInt) (PR #7525 fixes it) and std.numeric.gcd!T(T, T) when instantiated for BigInt. Adding special handling for x & 1 will aside from helping naive uses also improve performance of generic templated code that operates on arbitrary number-like types.

[dlang-bot]

Thanks for your pull request, @n8sh!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20980	enhancement	std.bigint.BigInt: special case x & non-negative int to avoid unnecessary allocation

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7544"

[n8sh]

Generalized for non-negative built-in integers.

[thewilsonator]

@n8sh please rebase it seems CI is stuck