Add support for OpenEphemeral bytecode

[diegoreis42]

First step to close #741. It allows creating temporary table operations and improve flexibility in handling in-memory data. Since is my first time working on core code, every feedback is welcome. @jussisaurio @pereman2 @PThorpe92 @penberg

[penberg]

Hey @diegoreis42, looks very sensible to me!

[PThorpe92]

This macro will probably have to be renamed?

[diegoreis42]

must_be_general_cursor maybe?

[diegoreis42]

I'm not so sure what should be returned here.

[jussisaurio]

This doesn't need to be a Rc<RefCell<T>> right? Just table: EphemeralTable

[jussisaurio]

We should support the key being either a rowid or an arbitrary key. From SQLite docs:

Open a new cursor P1 to a transient table. The cursor is always opened read/write even if the main database is read-only. The ephemeral table is deleted automatically when the cursor is closed.
If the cursor P1 is already opened on an ephemeral table, the table is cleared (all content is erased).

P2 is the number of columns in the ephemeral table. The cursor points to a BTree table if P4==0 and to a BTree index if P4 is not 0. If P4 is not NULL, it points to a KeyInfo structure that defines the format of keys in the index.

So I think EphemeralTable should be something like

pub struct EphemeralTable<Key: Ord> {
    pub rows: BTreeMap<Key, Vec<OwnedValue>>,
    pub next_rowid: u64,                      // generate rowids
    pub columns: Vec<Column>,                 // columns
}

OR define it as an enum:

enum Ephemeral {
  Table(EphemeralTable)
  Index(EphemeralIndex)
}

and have separate definitions.

Either way, searching with an index key should be O(log n) in an ephemeral index, instead of O(n) as here.