Mutually recursive type resolution
The protobuf syntax allows the definition of mutually recursive types, however this declaration is implicit (ie no dedicated syntax to group together types which are mutually recursive).
OCaml requires mutually recursive types to be defined with an explicit syntax (using the and keyword), therefore it is necessary to find all the mutually recursive protobuf types.
This module performs the recursion analysis using the Tarjan graph algorithm to find all the Strongly Connnected Components.
module Tt = Pb_typing_type_treeval group :
Pb_field_type.resolved Tt.proto ->
Pb_field_type.resolved Tt.proto listgroup types returns the list of all the mutually recursive group of types in reverse order of dependency. In other the last group of types of the returned list don't depend on any other types.