Check functions exist and parameters match

[Daniel Hughes]

We are only checking generic function syntax, which knowing anything about the actual functions being called.

This is enough to check the tables/columns referenced exist but not that the functions are being called correctly.

We should support all the built in sqlite functions. Users can also define functions using the c api, but it's probably safe to assume if they are using SqlSrcGen they don't have any of those. We could allow unknown functions but validate known ones, however this won't protect against typos in function names. We could also have the user tell us about user defined functions via Attributes.