Remove warnings

Trust/Vendors/BigInt/SMP Core.swift

@@ -2063,7 +2063,7 @@ public struct BDouble:
 
 		if let exp = nStr[Character("e")]
 		{
-			let beforeExp = String(nStr[0..<exp].characters.filter{ $0 != "." })
+			let beforeExp = String(nStr[0..<exp].filter{ $0 != "." })
 			var afterExp = nStr[(exp + 1)..<nStr.count]
 			var sign = false
 
@@ -2314,9 +2314,9 @@ public extension String
 	/// If possible, returns index of first ocurrence of char.
 	subscript(char: Character) -> Int?
 	{
-		if let idx = self.characters.index(of: char)
+		if let idx = self.index(of: char)
 		{
-			return self.characters.distance(from: self.startIndex, to: idx)
+			return self.distance(from: self.startIndex, to: idx)
 		}
 		return nil
 	}

Trust/Vendors/BigInt/SMP String Module.swift

@@ -31,7 +31,7 @@ private func convert(number: String, fromBase: Int, toBase: Int) -> String
 	var sum = BInt(0)
 	var multiplier = BInt(1)
 
-	for char in number.characters.reversed()
+	for char in number.reversed()
 	{
 		if let digit = chars.index(of: String(char))
 		{
@@ -136,7 +136,7 @@ private func digitsToString(_ digits: Digits) -> String
 	var i = digits.count - 2
 	while i >= 0
 	{
-		var str = String(digits[i])
+		let str = String(digits[i])
 
 		var paddingZeros = String(
 			repeating: "0",