[#212] Implemented Chain operator in query engine metta parser

[andre-senna]

Resolves #212

Final PR in the scope of #212 implementing the changes in the Querey Engine's MeTTa parser. Now Chain operator can be used in any query either as tokens or as metta expressions.

There are examples in pattern_matching_query_test.cc but the MeTTa syntax for Chain is like this:

tokens_query = {
        "CHAIN", "1", "1", "2",
            "NODE", "Symbol", "\"chimp\"",
            "NODE", "Symbol", "\"ent\"",
            "AND", "2",
                "OR", "2",
                    "LINK_TEMPLATE", "Expression", "3",
                        "NODE", "Symbol", "Inheritance",
                        "VARIABLE", "v1",
                        "NODE", "Symbol", "\"mammal\"",
                    "LINK_TEMPLATE", "Expression", "3",
                        "NODE", "Symbol", "Inheritance",
                        "VARIABLE", "v2",
                        "NODE", "Symbol", "\"mammal\"",
                "LINK_TEMPLATE", "Expression", "3",
                    "NODE", "Symbol", "Similarity",
                    "VARIABLE", "v1",
                    "VARIABLE", "v2"
};
    
metta_query =  "(chain 1 1 2 \"chimp\" \"ent\" (and (or (Inheritance $v1 \"mammal\") (Inheritance $v2 \"mammal\")) (Similarity $v1 $v2)))";

[ccgsnet]

LGTM (nitpicks only)