[refactor] typing/predef: introduce a variant of predefined type constructors

[gasche]

predef.ml contains the definition of all "predefined" type constructors, that are defined in the compiler rather than the standard library. Some type analyses need special cases for those predefined types, whose property often cannot be defined from their definition.

Currently the check for these special cases is done with an or-cascade of boolean checks, for example Typeopt.classify (for compilation of lazy thunks):

ocaml/typing/typeopt.ml

Lines 92 to 101 in d753ea1

	| Tconstr (p, _args, _abbrev) ->
	if Path.same p Predef.path_float then Float
	else if Path.same p Predef.path_lazy_t then Lazy
	else if Path.same p Predef.path_string
	|| Path.same p Predef.path_bytes
	|| Path.same p Predef.path_array
	|| Path.same p Predef.path_nativeint
	|| Path.same p Predef.path_int32
	|| Path.same p Predef.path_int64 then Addr
	else begin

This is not very robust, because no one remembers to update those analyses when new predefined types are introduced. And of course, the code I quoted above does not contain the logic one could expect for the predefined types floatarray, eff and continuation.

To make this code more robust, we add in typing/Predef a variant type of all predefined type constructors (it is split in two classes, the "abstract" type constructors and the "data" type constructors that have a datatype definition, because analyses typically only need to special-case the abstract ones). They are used just enough in the module to force people to add a new variant each time they add a predefined type.

The PR also includes a find_type_constr function to decide whether am arbitrary type constructor is predefined, and uses it to update the definition in Typeopt.classify -- which revealed the missing cases.