Add metadata_children function for accessing metadata tree

Author: bowenszhu

src/types.jl

@@ -923,6 +923,10 @@ end
 metadata(s::Symbolic) = s.metadata
 metadata(s::Symbolic, meta) = Setfield.@set! s.metadata = meta
 
+function metadata_children(s::BasicSymbolic)
+    s.meta.children
+end
+
 function hasmetadata(s::Symbolic, ctx)
     metadata(s) isa AbstractDict && haskey(metadata(s), ctx)
 end