exercises: use usize, not isize, when guaranteed non-negative (#207)

Rationale:

- This makes it self-documenting when the output cannot be negative.

- This makes it self-documenting when a negative input is not
  meaningful/useful, and when the user doesn't need to return an error
  for a negative input.

- There isn't much benefit for the user to change `isize` to `usize`
  themselves.

exercises/practice/collatz-conjecture/.meta/example.zig

@@ -2,12 +2,12 @@ pub const ComputationError = error{
     IllegalArgument,
 };
 
-pub fn steps(start: isize) ComputationError!isize {
+pub fn steps(start: isize) ComputationError!usize {
     if (start <= 0) {
         return ComputationError.IllegalArgument;
     }
     var number = start;
-    var count: isize = 0;
+    var count: usize = 0;
     while (number > 1) {
         if (@mod(number, 2) == 0) {
             number = @divTrunc(number, 2);

exercises/practice/darts/.meta/example.zig

@@ -12,7 +12,7 @@ pub const Coordinate = struct {
         };
     }
 
-    pub fn score(self: Coordinate) isize {
+    pub fn score(self: Coordinate) usize {
         const d = math.sqrt(self.x_coord * self.x_coord +
             self.y_coord * self.y_coord);
         if (d > 10) {

exercises/practice/darts/darts.zig

@@ -6,7 +6,7 @@ pub const Coordinate = struct {
         _ = y_coord;
         @compileError("please implement the init method");
     }
-    pub fn score(self: Coordinate) isize {
+    pub fn score(self: Coordinate) usize {
         _ = self;
         @compileError("please implement the score method");
     }

exercises/practice/difference-of-squares/.meta/example.zig

@@ -1,12 +1,12 @@
-pub fn squareOfSum(number: isize) isize {
+pub fn squareOfSum(number: usize) usize {
     const result = @divExact(number * (number + 1), 2);
     return result * result;
 }
 
-pub fn sumOfSquares(number: isize) isize {
+pub fn sumOfSquares(number: usize) usize {
     return @divExact(number * (number + 1) * (2 * number + 1), 6);
 }
 
-pub fn differenceOfSquares(number: isize) isize {
+pub fn differenceOfSquares(number: usize) usize {
     return squareOfSum(number) - sumOfSquares(number);
 }

exercises/practice/difference-of-squares/difference_of_squares.zig

@@ -1,14 +1,14 @@
-pub fn squareOfSum(number: isize) isize {
+pub fn squareOfSum(number: usize) usize {
     _ = number;
     @compileError("compute the sum of i from 0 to n then square it");
 }
 
-pub fn sumOfSquares(number: isize) isize {
+pub fn sumOfSquares(number: usize) usize {
     _ = number;
     @compileError("compute the sum of i^2 from 0 to n");
 }
 
-pub fn differenceOfSquares(number: isize) isize {
+pub fn differenceOfSquares(number: usize) usize {
     _ = number;
     @compileError("compute the difference between the square of sum and sum of squares");
 }

exercises/practice/resistor-color-duo/.meta/example.zig

@@ -11,6 +11,6 @@ pub const ColorBand = enum(u4) {
     white,
 };
 
-pub fn colorCode(colors: [2]ColorBand) isize {
-    return @as(isize, @enumToInt(colors[0])) * 10 + @as(isize, @enumToInt(colors[1]));
+pub fn colorCode(colors: [2]ColorBand) usize {
+    return @as(usize, @enumToInt(colors[0])) * 10 + @as(usize, @enumToInt(colors[1]));
 }

exercises/practice/resistor-color-duo/resistor_color_duo.zig

@@ -1,4 +1,4 @@
-pub fn colorCode(colors: [2]ColorBand) isize {
+pub fn colorCode(colors: [2]ColorBand) usize {
     _ = colors;
     @compileError("please implement the colorCode function");
 }

exercises/practice/resistor-color/.meta/example.zig

@@ -16,7 +16,7 @@ const band_colors = [_]ColorBand{
     .green, .blue,  .violet, .grey,   .white,
 };
 
-pub fn colorCode(color: ColorBand) isize {
+pub fn colorCode(color: ColorBand) usize {
     return @enumToInt(color);
 }
 

exercises/practice/resistor-color/resistor_color.zig

@@ -1,4 +1,4 @@
-pub fn colorCode(color: ColorBand) isize {
+pub fn colorCode(color: ColorBand) usize {
     @compileError("determine the value of a colorband on a resistor");
 }