Improve DateTime{Offset} "r" and "o" formatting performance

Two main changes:
1. Rewrote the formatting to use span, only to then discover that we already had almost exactly the same implementation in Utf8Formatter.  As that one had some extra optimizations around JIT behaviors, I ported that over instead.
2. Avoided [ThreadStatic] lookups unless necessary.

ToString/TryFormat for "o"/"O" improve by ~2.5x.

ToString/TryFormat for "r"/"R" improve by ~3x.

src/mscorlib/shared/System/DateTime.cs

@@ -1255,46 +1255,46 @@ internal DateTime ToLocalTime(bool throwOnOverflow)
 
         public String ToLongDateString()
         {
-            return DateTimeFormat.Format(this, "D", DateTimeFormatInfo.CurrentInfo);
+            return DateTimeFormat.Format(this, "D", null);
         }
 
         public String ToLongTimeString()
         {
-            return DateTimeFormat.Format(this, "T", DateTimeFormatInfo.CurrentInfo);
+            return DateTimeFormat.Format(this, "T", null);
         }
 
         public String ToShortDateString()
         {
-            return DateTimeFormat.Format(this, "d", DateTimeFormatInfo.CurrentInfo);
+            return DateTimeFormat.Format(this, "d", null);
         }
 
         public String ToShortTimeString()
         {
-            return DateTimeFormat.Format(this, "t", DateTimeFormatInfo.CurrentInfo);
+            return DateTimeFormat.Format(this, "t", null);
         }
 
         public override String ToString()
         {
-            return DateTimeFormat.Format(this, null, DateTimeFormatInfo.CurrentInfo);
+            return DateTimeFormat.Format(this, null, null);
         }
 
         public String ToString(String format)
         {
-            return DateTimeFormat.Format(this, format, DateTimeFormatInfo.CurrentInfo);
+            return DateTimeFormat.Format(this, format, null);
         }
 
         public String ToString(IFormatProvider provider)
         {
-            return DateTimeFormat.Format(this, null, DateTimeFormatInfo.GetInstance(provider));
+            return DateTimeFormat.Format(this, null, provider);
         }
 
         public String ToString(String format, IFormatProvider provider)
         {
-            return DateTimeFormat.Format(this, format, DateTimeFormatInfo.GetInstance(provider));
+            return DateTimeFormat.Format(this, format, provider);
         }
 
         public bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider provider = null) =>
-            DateTimeFormat.TryFormat(this, destination, out charsWritten, format, DateTimeFormatInfo.GetInstance(provider));
+            DateTimeFormat.TryFormat(this, destination, out charsWritten, format, provider);
 
         public DateTime ToUniversalTime()
         {

src/mscorlib/shared/System/DateTimeOffset.cs

@@ -755,26 +755,26 @@ internal DateTimeOffset ToLocalTime(bool throwOnOverflow)
 
         public override String ToString()
         {
-            return DateTimeFormat.Format(ClockDateTime, null, DateTimeFormatInfo.CurrentInfo, Offset);
+            return DateTimeFormat.Format(ClockDateTime, null, null, Offset);
         }
 
         public String ToString(String format)
         {
-            return DateTimeFormat.Format(ClockDateTime, format, DateTimeFormatInfo.CurrentInfo, Offset);
+            return DateTimeFormat.Format(ClockDateTime, format, null, Offset);
         }
 
         public String ToString(IFormatProvider formatProvider)
         {
-            return DateTimeFormat.Format(ClockDateTime, null, DateTimeFormatInfo.GetInstance(formatProvider), Offset);
+            return DateTimeFormat.Format(ClockDateTime, null, null, Offset);
         }
 
         public String ToString(String format, IFormatProvider formatProvider)
         {
-            return DateTimeFormat.Format(ClockDateTime, format, DateTimeFormatInfo.GetInstance(formatProvider), Offset);
+            return DateTimeFormat.Format(ClockDateTime, format, null, Offset);
         }
 
         public bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider formatProvider = null) =>
-            DateTimeFormat.TryFormat(ClockDateTime, destination, out charsWritten, format, DateTimeFormatInfo.GetInstance(formatProvider), Offset);
+            DateTimeFormat.TryFormat(ClockDateTime, destination, out charsWritten, format, formatProvider, Offset);
 
         public DateTimeOffset ToUniversalTime()
         {