exercises: use new for loop syntax (#312)

Zig 0.11.0 added new for loop syntax [1]. Prefer to loop over a range,
which avoids some variables.

For more discussion, see Loris' blog post [2].

[1] https://ziglang.org/download/0.11.0/release-notes.html#Multi-Object-For-Loops
[2] https://kristoff.it/blog/zig-multi-sequence-for-loops/

Refs: #313

exercises/practice/acronym/.meta/example.zig

@@ -6,8 +6,7 @@ pub fn abbreviate(allocator: mem.Allocator, words: []const u8) mem.Allocator.Err
     var letters = std.ArrayList(u8).init(allocator);
     defer letters.deinit();
 
-    var i: usize = 0;
-    while (i < words.len) : (i += 1) {
+    for (0..words.len) |i| {
         if (!ascii.isAlphabetic(words[i])) continue;
 
         if (i == 0 or words[i - 1] == ' ' or words[i - 1] == '-' or words[i - 1] == '_') {

exercises/practice/dnd-character/.meta/example.zig

@@ -5,9 +5,8 @@ const random = prng.random();
 
 pub fn ability() u8 {
     var lowest: u8 = std.math.maxInt(u8);
-    var i: usize = 0;
     var result: u8 = 0;
-    while (i < 4) : (i += 1) {
+    for (0..4) |_| {
         const roll = random.intRangeAtMost(u8, 1, 6);
         result += roll;
         lowest = @min(lowest, roll);

exercises/practice/dnd-character/test_dnd_character.zig

@@ -105,8 +105,7 @@ fn isValidAbilityScore(n: isize) bool {
 }
 
 test "random ability is within range" {
-    var i: usize = 0;
-    while (i < 20) : (i += 1) {
+    for (0..20) |_| {
         const actual = dnd_character.ability();
         try testing.expect(isValidAbilityScore(actual));
     }
@@ -123,8 +122,7 @@ fn isValid(c: Character) bool {
 }
 
 test "random character is valid" {
-    var i: usize = 0;
-    while (i < 20) : (i += 1) {
+    for (0..20) |_| {
         const character = Character.init();
         try testing.expect(isValid(character));
     }

exercises/practice/perfect-numbers/.meta/example.zig

@@ -11,9 +11,8 @@ pub const Classification = enum {
 fn aliquotSum(n: u64) u64 {
     if (n == 1) return 0;
     var result: u64 = 1;
-    var i: usize = 2;
     const isqrt_n = std.math.sqrt(n);
-    while (i <= isqrt_n) : (i += 1) {
+    for (2..isqrt_n + 1) |i| {
         if (n % i == 0) result += i + (n / i);
     }
     // When `n` is a square number, we added the same divisor twice inside the loop.

exercises/practice/proverb/.meta/example.zig

@@ -6,8 +6,7 @@ pub fn recite(allocator: mem.Allocator, words: []const []const u8) (fmt.AllocPri
     var main_slice = try allocator.alloc([]u8, words.len);
 
     if (words.len > 1) {
-        var i: usize = 0;
-        while (i < words.len - 1) : (i += 1) {
+        for (0..words.len - 1) |i| {
             main_slice[i] = try fmt.allocPrint(allocator, "For want of a {s} the {s} was lost.\n", .{ words[i], words[i + 1] });
         }
     }