Modularise the implementation

Minor

• symbol is now part of the sql object, so every extension could use it to validate the sql object against it. Currently only sql.query make use of this (#44)

Patch

• Modularised every logical component, like the main helper functions, the manipulation methods, the selection methods and the tag helpers, to have it's own file (#44)
• Refactored sql.insert to align with the other manipulation methods only have one sql.query for the different use cases (#44)