isPowerOf2 should not return true for signed min

[oscbyspro]

From oscbyspro/Numberick#32

I noticed that isPowerOf2 fails when Self.isSigned && self == min:

@inlinable public var isPowerOf2: Bool {
    self.nonzeroBitCount == 1 // ⚠️
}

@inlinable public var isPowerOf2: Bool {
    !self.isLessThanZero && self.nonzeroBitCount == 1 // ✅
}