Rework sign and magnitude init

[oscbyspro]

Introduction

Numberick integers have these methods and they kind of suck:

static func exactly (sign: FloatingPointSign, magnitude: Magnitude) -> Self?
static func clamping(sign: FloatingPointSign, magnitude: Magnitude) -> Self

Proposal

It would be better if NBKBinaryInteger required the following instead:

init?(sign: FloatingPointSign, magnitude: Magnitude)

Motivation

• the clamping method is trivial and not used anywhere
• the fallible method does not match IntXL's init(sign:magnitude:) (NBKFlexibleWidthKit #33)
• nonfallible initializers satisfy fallible protocol requirements but this is not true for methods
• Swift propagates initialization failures but does not do the same for methods

A) self.init(sign: sign, magnitude: magnitude)

B) if let x = Self.exactly(sign: sign, magnitude: magnitude) { self = x } else { return nil }