StorageExhausted holds over the Map's four allocating operations

One rule over every allocating operation, so it has to hold for map-new,
put, reserve and clone exactly as it holds for vec-new, push, reserve
and clone. put stays Unit and clone stays the container; nothing grows a
Result.

A map is the harder of the two and that is why it gets its own program.
A Vec's failing allocation leaves the Vec untouched, whereas a map's
growth allocates a whole new block, rehashes into it and only then
releases the old one — so a failure partway has to leave the map exactly
as it was or the retry re-attempts against a half-moved map. 300 entries
through several grows against a ceiling that is raised each time, then
every one of them read back: no entry lost, none doubled.

test/programs/map-exhausted.flan

@@ -0,0 +1,82 @@
+;;;; StorageExhausted and retry, over a Map — spec-memory.md, "Allocation
+;;;; failure". The rule is one rule over *every* allocating operation, so it has
+;;;; to hold for map-new, put, reserve and clone exactly as exhausted.flan shows
+;;;; it holding for vec-new, push, reserve and clone. put stays Unit, clone
+;;;; stays the container, and no signature anywhere grows a Result.
+;;;;
+;;;; A map is the harder case of the two, and that is why it gets its own
+;;;; program. A Vec's failing allocation leaves the Vec untouched. A map's
+;;;; growth allocates a whole new block, rehashes into it and only then
+;;;; releases the old one — so a failure partway must leave the map exactly as
+;;;; it was, or the retry re-attempts against a half-moved map.
+
+;; Globals, because a handler cannot see the locals of the function that
+;; established it.
+(defvar tight Allocator)
+(defvar failures i64)
+(defvar last-bytes i64)
+(defvar same-allocator bool)
+
+(defn main [] i32
+  (set tight (heap-allocator))
+  ;; Enough for the smallest block and not for the one after it. A map's block
+  ;; is keys, values, hashes and scratch, each rounded to a cache line, so it
+  ;; is a few hundred bytes at eight slots — the ceiling has to be low enough
+  ;; to be hit and high enough that the first block fits after one raise.
+  (set-alloc-budget tight 64)
+
+  (handler-bind
+    [(StorageExhausted [c]
+       (set failures (+ failures 1))
+       (set last-bytes (.bytes c))
+       (set same-allocator (= (.allocator c) (alloc-id tight)))
+       ;; Raise the ceiling and re-attempt the same request. The map is
+       ;; untouched and its allocator is unchanged, which is why this can
+       ;; succeed — releasing the region instead would invalidate the map,
+       ;; which is what the epoch check catches and is its own program.
+       (set-alloc-budget tight (* 4 (alloc-budget tight)))
+       (invoke-restart 'retry))]
+    (let [m (map-new i32 i64 tight)]
+      ;; Several grows, each one a fresh block rehashed into. A put that fails
+      ;; puts nothing, and the retry puts exactly once — so no entry is lost
+      ;; and none is doubled.
+      (dotimes [i 300] (put m i (* (i64 i) 7)))
+      (println (len m))                 ; 300
+      (let [bad 0]
+        (dotimes [i 300]
+          (match (get m i)
+            (Some v) (if (not (= v (* (i64 i) 7))) (set bad (+ bad 1)))
+            None (set bad (+ bad 1))))
+        (println bad))                  ; 0
+      (free m)))
+
+  (println (> failures 1))              ; true
+  (println (> last-bytes 0))            ; true
+  (println same-allocator)              ; true
+
+  ;; reserve asks for the whole block at once, and clone asks the new allocator
+  ;; for one big enough to hold the source — both are allocating operations and
+  ;; neither returns an error.
+  (set-alloc-budget tight 64)
+  (set failures 0)
+  (handler-bind
+    [(StorageExhausted [c]
+       (set failures (+ failures 1))
+       (set-alloc-budget tight (* 8 (alloc-budget tight)))
+       (invoke-restart 'retry))]
+    (let [m (map-new string i32 tight)]
+      (reserve m 200)
+      (println (len m))                 ; 0
+      (put m "a" 1)
+      (put m "b" 2)
+      (let [w (clone m)]
+        (println (len w))               ; 2
+        (match (get w "b")
+          (Some v) (println v)          ; 2
+          None (println "missing"))
+        (free w))
+      (free m)))
+  (println (> failures 0))              ; true
+
+  (set-alloc-budget tight 0)
+  0)