Improve code organization

[bgrainger]

The current code layout has evolved in a somewhat haphazard way. To make it more internally consistent and easier for new contributors to get involved, I propose reorganising it as follows:

• MySqlConnector
  • MySql.Data.MySqlClient - only types necessary to clone Connector/NET public API
  • Protocol - enums, constants, etc. defined in the MySQL protocol
    • Payloads - types representing individual payloads
    • Serialization - reading/writing packets & payloads
  • ? - ADO.NET implementation helpers, MySqlSession, MySqlParser, MySqlXaTransaction, etc.
  • Utility - general-purpose code, extension methods

[bgrainger]

It would be good if the reorganisation could make it easier to expose a "direct" API (that corresponds much more closely to the underlying MySQL protocol) for clients who need high-performance access but don't need/want ADO.NET.