[C#] Add ToString (#853)

* [C#] Add more string reading/writing methods to DirectBuffer
More, hopefully efficient, mechanisms to read and write strings to and from the DirectBuffer.

* [C#] Reverted to .net45

* [C#] Reverted to .net45

* [C#] Updated Samples to use the new String methods

* [C#] Fixed checkstyle

* [C#] Cleaning

* Added Modified Benchmarks\nBoth a modified version of the the CarBenchmark which encodes and decodes to/from string; and a version wich uses the new methods to encode and decode strings

* Added ToString

* Added support for contant tokens

* Fixed issue with ToString of tokens without an encoding - like raw data fields

* Fixed bracing issue in generated CSharp code. Also Fixed an indent issue.

Co-authored-by: Rob Purdy <rob.purdy@iongroup.com>

Author: MFrpurdy