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

src/barabadb/query/ast.nim

@@ -43,6 +43,10 @@ type
     nkSetVar
     nkCreateGraph
     nkDropGraph
+    nkCreateDatabase
+    nkDropDatabase
+    nkUseDatabase
+    nkShowDatabases
 
     # Clauses
     nkFrom
@@ -336,6 +340,16 @@ type
     of nkDropGraph:
       dgName*: string
       dgIfExists*: bool
+    of nkCreateDatabase:
+      cdDbName*: string
+      cdIfNotExists*: bool
+    of nkDropDatabase:
+      ddDbName*: string
+      ddIfExists*: bool
+    of nkUseDatabase:
+      udDbName*: string
+    of nkShowDatabases:
+      discard
     of nkApplyMigration:
       amName*: string
     of nkMigrationStatus: