Develop the conversion of Datalog rules into operation-based SQL queries

[gfngfn]

This PR adds the following function to Ast2sql:

val convert_expr_to_operation_based_sql : Expr.expr -> (sql_operation list, error) result

This function takes the following arguments:

• expr : Expr.expr: Datalog rules and sources for conversion

and then converts expr.rules to the corresponding operation-based SQL queries. Rules that contain NumberedVar or AggVar are currently not supported, though.

The present PR also adds unit tests for the function above as src/test/ast2sql_operation_based_conversion_test.ml. One can run these tests by invoking:

$ make test

In order to confirm that convert_expr_to_operation_based_sql works as intended, please see:

1. the unit tests, and
2. the following results of operation check:
   • https://gist.github.com/gfngfn/b3777fa4175615280a2b00ab29ad7b19 .

I would appreciate it if you could give any comments or suggestion.

[yoshihiro503]

• What is AS inst? Is it necessary?
• rule1 means add to ed, so the corresponding query1 must be INSERT INTO ed, same for temp2 and DELETE statement.
• The DELETE statement didn't work in the original spec, if it existed, it would all disappear, what I want to do is something like eed - temp1, can SQL do something with the DELETE WHERE clause (instead of EXISTS)?

[gfngfn]

Thank you all for pointing out!

Firstly c1f9657 remedies the second problem.

[gfngfn]

205e15a remedies the first issue (i.e. removes unnecessary AS insts).

[gfngfn]

06c3b1a fixes the third issue, i.e., makes the program output DELETE queries appropriate in operational terms (which do not seem so performant though).