Implements built-in window functions

[geoffmacd]

Adds window functions to Sqlite.swift, see https://www.sqlite.org/windowfunctions.html#built_in_window_functions. The OVER cause was problematic in the current structure of the code, so for now, only OVER (ORDER BY {expression}) is supported but that is most of the use cases. The ORDER BY used here is not the same as the func order() in QueryType and cannot used like this.

• row_number()
• rank()
• dense_rank()
• percent_rank()
• cume_dist()
• ntile(N)
• lag(expr, offset, default)
• lead(expr, offset, default)
• first_value(expr)
• last_value(expr)
• nth_value(expr, N)