Idea: dynamic table-valued functions, backed by WebAssembly user-defined functions

Table-valued functions are described here: https://www.sqlite.org/vtab.html#tabfunc2

Now that we support dynamic user-defined functions in WebAssembly, we should consider going a step further and allowing users to dynamically create table-valued functions, using (and maybe extending) the existing CREATE VIRTUAL TABLE interface. The rough idea is to simply have a functor that keeps its state in a virtual table, and uses a user-defined function to generate data.

Tribute to @fire for coming up with the idea and patiently explaining it to me multiple times until I grasped it :innocent: