batch some binding changes

[vtjnash]

For model simplicity (and eventually for better unspecialized compilation), try to backdate initial module operations back to world 0 if nobody could have observed them before, rather than putting each incremental operation in a separate, but unobserved (and unobservable) world. The actual change I was pursuing with this I will put up as a separate PR, and since it saves about 30 seconds on the bootstrapping but might need some extra work (it is more of a semantic change than the other changes here):

diff --git a/src/module.c b/src/module.c
index 214124f0835..814a4ac9cc2 100644
--- a/src/module.c
+++ b/src/module.c
@@ -1220,6 +1220,13 @@ int jl_module_public_(jl_module_t *from, jl_sym_t *s, int exported, size_t new_w
     }
     jl_atomic_fetch_or_relaxed(&b->flags, BINDING_FLAG_PUBLICP);
     if (was_exported != exported) {
+        if (from->usings_backedges == jl_nothing && !jl_object_in_image((jl_value_t*)from)) {
+            // until the first `using` of this module, simply backdate any export statements
+            // to apply to the current binding instead of making a new one
+            check_safe_newbinding(from, s);
+            bpart->kind |= PARTITION_FLAG_EXPORTED;
+            return 1;
+        }
         jl_replace_binding_locked2(b, bpart, bpart->restriction, bpart->kind | PARTITION_FLAG_EXPORTED, new_world);
         return 1;
     }