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/lexer.nim

@@ -160,6 +160,10 @@ type
     tkBfs
     tkDfs
     tkPath
+    tkDatabase
+    tkDatabases
+    tkUse
+    tkShow
 
     tkAutoIncrement
     tkSequence
@@ -382,6 +386,10 @@ const keywords*: Table[string, TokenKind] = {
   "bfs": tkBfs,
   "dfs": tkDfs,
   "path": tkPath,
+  "database": tkDatabase,
+  "databases": tkDatabases,
+  "use": tkUse,
+  "show": tkShow,
   "over": tkOver,
   "partition": tkPartition,
   "row": tkRow,