Fixed bugs reported by the mycology test suite

src/field.zig

@@ -287,7 +287,7 @@ pub const Field = struct {
                     }
                 }
                 switch (buffer[i]) {
-                    12 => x += 1,
+                    12 => continue,
                     '\r' => {
                         x = 0;
                         y += 1;

src/main.zig

@@ -12,7 +12,7 @@ pub fn main() anyerror!void {
         std.os.exit(1);
     }
 
-    var file = try std.fs.cwd().openFile("mycology/sanity.bf", .{});
+    var file = try std.fs.cwd().openFile(args[1], .{});
     defer file.close();
 
     const env: []const [*:0]const u8 = std.os.environ;

src/stack.zig

@@ -12,9 +12,8 @@ pub const Stack = struct {
         self.allocator.destroy(self);
     }
     pub fn duplicate(self: *Stack) !void {
-        if (self.data.items.len > 0) {
-            try self.push(self.data.items[self.data.items.len - 1]);
-        }
+        const v = self.pop();
+        try self.pushVector([2]i64{ v, v });
     }
     test "duplicate" {
         var s = try Stack.init(std.testing.allocator);