Move Back into Statements the Core Features

[tsutomi]

In a previous iteration of the system, the core functionalities of the language (eg. the insertion of data in a table, the creation of a table, the select from a table) have been made available through extension methods to the core system.
This design was thought in a logic of a code-centric architecture, where statements were supposed to be just invoke points of other code.

The limitation of this design is emerging with the increasing number of statements and special cases to be handled by each statement of the language.

The idea is to move back to the statement class implementation the execution of the features, and make the extension classes to construct (and dispose) the statements at invoke. Basically, invert the current design