No minLastIndex[...] methods

[oscbyspro]

From oscbyspro/Numberick#11

The method minLastIndexReportingIsZeroOrMinusOne() is only used once.

/// Returns the last index in reduced two's complement form, along with an
/// indicator of whether this value is zero or minus one. In the case where
/// this value is zero or minus one, the index is zero.
///
@inlinable public func minLastIndexReportingIsZeroOrMinusOne() -> (minLastIndex: Int, isZeroOrMinusOne: Bool) {
    let sign  = UInt(repeating: self.isLessThanZero)
    let index = self.lastIndex(where:{ $0 != sign })
    return index.map({( $0, false )}) ?? (Int.zero, true)
}

In the unsigned case it's almost identical to the following expression:

let minLastIndex = self.lastIndex(where:{ !$0.isZero }) ?? self.startIndex

I'll remove it because it's A) too simple and B) its semantics may not always be what you want. In some unsigned cases you might prefer if zero is considered empty. This becomes much more apparent if you consider methods like minBitWidth().