PRIMARY KEY auto-index is not populated during insert

[sivukhin]

PRIMARY KEY auto-index now always empty in Limbo:

limbo> CREATE TABLE users (id PRIMARY KEY, username TEXT);
limbo> INSERT INTO users VALUES (1, 'alice');
limbo> SELECT * FROM users WHERE id = 1;
#[nothing here]

The problem is that during INSERT planner completely ignore auto-index:

limbo> EXPLAIN INSERT INTO users VALUES (1, 'alice');
addr  opcode             p1    p2    p3    p4             p5  comment
----  -----------------  ----  ----  ----  -------------  --  -------
0     Init               0     10    0                    0   Start at 10
1     OpenWriteAsync     0     2     0                    0   
2     OpenWriteAwait     0     0     0                    0   
3     Integer            1     2     0                    0   r[2]=1
4     String8            0     3     0     alice          0   r[3]='alice'
5     NewRowId           0     1     0                    0   
6     MakeRecord         2     2     4                    0   r[4]=mkrec(r[2..3])
7     InsertAsync        0     4     1                    0   
8     InsertAwait        0     0     0                    0   
9     Halt               0     0     0                    0   
10    Transaction        0     1     0                    0   
11    Goto               0     1     0                    0   

[jussisaurio]

we don't have any index insertions yet :)

i made this ages ago but didnt pursue merging it because it needed a proper tree balancing to work correctly for any non trivial inputs

#665