backoff: Add an exponential backoff combinator

diff --git a/extra/backoff/authors.txt b/extra/backoff/authors.txt
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+++ b/extra/backoff/authors.txt
@@ -0,0 +1 @@
+Doug Coleman

diff --git a/extra/backoff/backoff.factor b/extra/backoff/backoff.factor
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+++ b/extra/backoff/backoff.factor
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+! Copyright (C) 2023 Doug Coleman.
+! See https://factorcode.org/license.txt for BSD license.
+USING: calendar combinators.short-circuit kernel math math.order
+shuffle threads ;
+IN: backoff
+
+: exponential-backoff-from ( quot: ( -- success? ) max-seconds max-count/f n -- )
+    2dup { [ drop ] [ <= ] } 2&& [
+        4drop
+    ] [
+        [ nip 2^ min seconds sleep call ] 4keep 5roll [
+            4drop
+        ] [
+            1 + exponential-backoff-from
+        ] if
+    ] if ; inline recursive
+
+: exponential-backoff-count ( quot: ( -- success? ) max-seconds max-count -- )
+    0 exponential-backoff-from ; inline
+
+: exponential-backoff ( quot: ( -- success? ) max-seconds -- )
+    f exponential-backoff-count ; inline