Fix `Export` order for `length`. Remove `length` hack in strings. Add a test.

tests/length.ref

+length : ∀ A : Type, list A → nat
+
+length is not universe polymorphic
+Arguments length {A}%type_scope _%list_scope
+length is transparent
+Expands to: Constant Coq.Init.Datatypes.length
+length : ∀ A : Type, list A → nat
+
+length is not universe polymorphic
+Arguments length {A}%type_scope _%list_scope
+length is transparent
+Expands to: Constant Coq.Init.Datatypes.length
+length : ∀ A : Type, list A → nat
+
+length is not universe polymorphic
+Arguments length {A}%type_scope _%list_scope
+length is transparent
+Expands to: Constant Coq.Init.Datatypes.length

tests/length.v

+(** Check that we always get the [length] function on lists, not on strings. *)
+From stdpp Require Import prelude.
+About length.
+From stdpp Require Import strings.
+About length.
+From stdpp Require Import prelude.
+About length.
\ No newline at end of file

theories/base.v

@@ -3,7 +3,12 @@ that are used throughout the whole development. Most importantly it contains
 abstract interfaces for ordered structures, sets, and various other data
 structures. *)
 
-From Coq Require Export Morphisms RelationClasses List Bool Utf8 Setoid.
+(** The order of this [Require Export] is important: The definition of [length]
+in [List] should shadow the definition of [length] in [String]. We also need
+to export [Datatypes] because [List] contains a [parsing only] notation for
+[length], not the actual definition of [length], which is in [Datatypes]. *)
+From Coq Require Export String Datatypes List.
+From Coq Require Export Morphisms RelationClasses Bool Utf8 Setoid.
 From Coq Require Import Permutation.
 Set Default Proof Using "Type".
 Export ListNotations.

theories/strings.v

 From Coq Require Import Ascii.
-From Coq Require Export String.
 From stdpp Require Export list.
 From stdpp Require Import countable.
 Set Default Proof Using "Type".
 
-(* To avoid randomly ending up with String.length because this module is
-imported hereditarily somewhere. *)
-Notation length := List.length.
-
 (** * Fix scopes *)
 Open Scope string_scope.
 (* Make sure [list_scope] has priority over [string_scope], so that