Draft implementation for #650.

This PR adds a new src/kontrol/solidity folder that contains an antlr4 grammar for a subset of Solidity we want to use in NatSpec comments specifying preconditions.

It also adds a Precondition class (to be refactored further) that contains a to_kapply property, which translates the parsed string to a KApply term representing the constraint. For example, the following test passes

    /// @custom:kontrol-precondition x <= 14,
    function testPrecondition(uint256 x) public pure {
        assert(x <= 14);
    }

because the following constraint get added to the initial term:

#And ( { true #Equals VV0_x_114b9705:Int <Int 14 }