Add SingleOps.__round__ and optimizations in Builtin.round.

Testing
===
```
>>> import System
>>> v = 987.654321
>>> assert round(System.Double(v)) == round(System.Single(v))
>>> print(round(System.Double(v), 3), round(System.Single(v), 3))
987.654 987.654
```

(Attempted to add assertions in tests/suite/test_builtinfunc.py, but attempting to run fails with
```
RUNNING IRONPYTHON TESTS USING THESE FLAGS:
Traceback (most recent call last):
  File "run.py", line 330, in <module>
  File "run.py", line 295, in runTest
  File "run.py", line 241, in runTestSlow
  File "run.py", line 226, in run_one_command
AttributeError: 'module' object has no attribute 'popen3'
```
)

```

Author: mickp

src/core/IronPython/Modules/Builtin.cs

@@ -1313,6 +1313,10 @@ public static object repr(CodeContext/*!*/ context, object? o)
                 return DoubleOps.__round__(d);
             }
 
+            if (number is float f) {
+                return SingleOps.__round__(f);
+            }
+
             if (number is int i) {
                 return Int32Ops.__round__(i);
             }
@@ -1338,6 +1342,9 @@ public static object repr(CodeContext/*!*/ context, object? o)
                 if (number is double d) {
                     return DoubleOps.__round__(d, ndi);
                 }
+                if (number is float f) {
+                    return SingleOps.__round__(f, ndi);
+                }
                 if (number is int i) {
                     return Int32Ops.__round__(i, ndi);
                 }
@@ -1350,6 +1357,9 @@ public static object repr(CodeContext/*!*/ context, object? o)
                 if (number is double d) {
                     return DoubleOps.__round__(d, ndbi);
                 }
+                if (number is float f) {
+                    return SingleOps.__round__(f, ndbi);
+                }
                 if (number is int i) {
                     return Int32Ops.__round__(i, ndbi);
                 }

src/core/IronPython/Runtime/Operations/FloatOps.cs

@@ -1187,5 +1187,9 @@ public static int __hash__(float x) {
         public static double __float__(float x) {
             return x;
         }
+
+        public static object __round__(float self) => DoubleOps.__round__(self);
+
+        public static float __round__(float self, object ndigits) => (float)DoubleOps.__round__(self, ndigits);
     }
 }