Enhanced GCD that doesn't trap

By returning T.Magnitude instead of T, the gcd call never traps. This also allows the end user of the call to decide how to handle the result.

PR: https://github.com/apple/swift-numerics/pull/310

(Also note that this is in preparation for lcm, which will be a separate PR.)

Review PR #310 and the existing gcd implementation first, then check how its current return type and trapping behavior are covered. Done means gcd returns T.Magnitude without trapping and leaves result handling to the caller; lcm remains separate.

Written by the indexing model from the issue text.