Update SQL AST in accordance with the SQL spec

[joshua-spacetime]

The SqlAst should represent the SQL grammar as defined in the spec. As such there are a few changes we should make.

1. No name resolution or type checking

   We should have a purely syntactic representation. Type checking and name resolution should be done after we've derived this representation.

2. No column expressions

   Right now we have Column::UnnamedExpr(Expr), but we decided in the spec to only support column names, qualified and unqualified.

3. Errors

   We want good internal as well as user facing error messages. error-stack looks like a way to achieve the former, ariadne the latter. Now is a good time to experiment with both.

We should probably define a new AST type first. We can replace SqlAst with the new type after we've implemented type checking and name resolution for the new type.