Typesafe dynamic filtering/ordering/paging

The issue names no repository files or tests. Start by locating the SQL<...> generated-type path described here and trace how output columns are exposed. Done means eligible single-SELECT queries support type-safe By values, ordering, paging, and a command that preserves the applied clauses.

Right now there is no way to dynamically order queries.

This makes the library unsuitable for applications that let the user sort a paged table of records.
That's a lot of applications!

This should be solvable without giving up type safety. Here's the design I have in mind:

Any command that consists of a single SELECT statement which is not known to be a single-row SELECT should be dynamically orderable.

e.g.

type MyQuery = SQL<"select Column1, 1+1 as Column2 from SomeTable">

This means that it gets the following extra stuff in its generated type:

• A nested type MyQuery.By with a private constructor and an overrided ToString().
• Static getters MyQuery.By.Column1 and MyQuery.By.Column2, which return instances of MyQuery.By
• A static method MyQuery.By.OfString("Column1") which checks that the passed string is one of the statically known output column names. MyQuery.By.OfString(MyQuery.By.Column2.ToString()) should work.
• A static OrderBy method taking a MyQuery.By and a DU Ascending|Descending and returning a MyQuery.OrderedQuery
• MyQuery.OrderedQuery has an instance method ThenBy taking another by+direction, Page taking limit+offset
• MyQuery.OrderedQuery has an instance Command method which is like the regular MyQuery.Command(param1 = ...) method, but includes the orderings applied so far

Hypothetical usage:

type MyQuery = SQL<"select Column1, 1+1 as Column2 from SomeTable where Name like @name">

let example (conn : ConnectionContext) =
    MyQuery
        .OrderBy(MyQuery.By.Column1, Ascending)
        .ThenBy(MyQuery.By.Column2, Descending)
        .Page(25, 50)
        .Command(name = "%a%")
        .Execute(conn)

I think it would be OK to limit to one ThenBy clause. Orderings more complicated than that don't make much sense to generate dynamically.

Thoughts?