Python: collapse two-layer AstNodeImpl into a single Ast module

Merge the previous `Ast` and `AstSigImpl` modules into a single
`module Ast implements AstSig<Py::Location>`. Classes now use the
signature names (IfStmt, WhileStmt, ForeachStmt, etc.) and signature
predicates (getCondition, getThen, getElse, etc.) directly, with no
intermediate renaming layer.

Drop the TStmtListNode newtype branch entirely. Replace it with a
synthetic TBlockStmt(parent, slot) keyed by a parent AST node and a
slot label string ('body', 'orelse', 'finally'). Py::StmtList no
longer appears in the newtype; the BlockStmt class provides indexed
access to the underlying body items via getStmt(n).

All 22 of 24 evaluation-order tests still pass; the same 2
comprehension-related failures predate this refactor.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/ql/lib/printCfgNew.ql

@@ -31,7 +31,7 @@ module ViewCfgQueryInput implements ControlFlow::ViewCfgQueryInputSig<Py::File>
   predicate selectedSourceColumn = selectedSourceColumnAlias/0;
 
   predicate cfgScopeSpan(
-    AstSigImpl::Callable callable, Py::File file, int startLine, int startColumn, int endLine,
+    Ast::Callable callable, Py::File file, int startLine, int startColumn, int endLine,
     int endColumn
   ) {
     exists(Py::Scope scope |