fix(query,types): refactor Row to Value-typed map, fix IN list, nkPath, multi-table joins

- Refactored Row from Table[string, string] to Table[string, Value]
- Added Value operators: ==, !=, $, in for string interop
- Fixed valueToString for vkNull to return '\\N' instead of ''
- Fixed evalExpr irekField to return vkNull when field not found
- Added IN (val1, val2, ...) parser support
- Fixed nkPath column names in multi-table joins
- Fixed LATERAL JOIN null padding when no matching rows
- Added CREATE/DROP/USE/SHOW DATABASE parser support
- Adapted all tests for new Value type

tests/join_tests.nim

@@ -3,6 +3,8 @@ import std/os
 import std/strutils
 import std/monotimes
 import std/tables
+import std/sequtils
+import barabadb/core/types
 import barabadb/query/executor as qexec
 import barabadb/query/parser
 import barabadb/storage/lsm
@@ -97,7 +99,7 @@ suite "JOIN execution":
     var foundBob = false
     for row in r.rows:
       if row["u.name"] == "Bob":
-        check row["x.total"] == "\\N"
+        check row["x.total"].kind == vkNull
         foundBob = true
     check foundBob