Update all std.mem.split calls to their appropriate function

Everywhere that can now use `splitScalar` should get a nice little performance boost.

test/src/check-stack-trace.zig

@@ -27,7 +27,7 @@ pub fn main() !void {
         var buf = std.ArrayList(u8).init(arena);
         defer buf.deinit();
         if (stderr.len != 0 and stderr[stderr.len - 1] == '\n') stderr = stderr[0 .. stderr.len - 1];
-        var it = mem.split(u8, stderr, "\n");
+        var it = mem.splitScalar(u8, stderr, '\n');
         process_lines: while (it.next()) |line| {
             if (line.len == 0) continue;