Started to switch from member serialize -> non-member interpret

This will allow database connectors to specialize the interpretation of
the expression tree and interpret queries in vendor specific ways where
required.
Continued to implement standard serializing interpreters

Everything that is interpreted also has to move out of detail namespace,
because it might have to be specialized for a database connector
Removed superfluous template parameter from interpreter

The Db parameter would actually have prevented different Context
overloads (or made more difficult than necessary).
Added interpreter to select, select_flag_list, select_expression_list
interpreter for from()
interpret works for where()
interpret works for group_by(), having() und .like()
Cleaned up a hack in like()
interpret handles order_by(), limit() and offset()
Interpret works for parameters now

Also, paramters inherit the operators they need
interpret works for insert now

Also added explicit .default_values() method
Replaced implicit trivial_value_is_null tag for columns with explicit tvin method

This isn't database vendor specific, but library vendor specific, as I
used this implicit behaviour quite a lot, but it is utterly confusing
when mixed with prepared statements. Explicit is better here.
interpreter added for update()
interpret works for in()
Added interpret support for remove()
Added missing file
Added interpreters for a bunch of functions and table aliases
Cleaned up some outdated serialize functions
Added interpreters to is_null, multi_column, alias and select pseudo table
Added missing interpreter specializations, especially for serializable