Avoid intermediate strings in some StringBuilder formatting

[stephentoub]

Several appends to string builders are first creating strings and then appending those strings, and they're not using the existing Append(primitive) overloads because they want to customize the format string or provider as part of appending the value. This fixes a few of those cases, using another internal AppendSpanFormattable overload on StringBuilder.

Example:

private DateTime _dt = new DateTime(2019, 1, 20, 7, 37, 59, DateTimeKind.Utc);
private char[] _buffer = new char[1024];
[Benchmark] public bool DTTryFormat() => _dt.TryFormat(_buffer, out int charsWritten);

allocates 64 bytes before and 0 after.

cc: @jkotas, @tarekgh

[jkotas]

LGTM modulo one nit.