translate: add a new relabel-overlap option

Fixes issue #536.  Also a part of issue #500.

* spot/twaalgos/translate.hh, spot/twaalgos/translate.cc: Implement
this new option.
* bin/spot-x.cc, NEWS: Mention it.
* tests/core/ltl2tgba2.test: Add the test case from issue #536.

NEWS

@@ -37,6 +37,29 @@ New in spot 2.11.6.dev  (not yet released)
     replace boolean subformulas by fresh atomic propositions even if
     those subformulas share atomic propositions.
 
+  - spot::translate() has a new -x option "relabel-overlap=M" that
+    augments the existing "relabel-bool=N".  By default, N=4, M=8.
+    When the formula to translate has more than N atomic propositions,
+    relabel_bse() is first called to attempt to rename non-overlaping
+    boolean subexpressions (i.e., no shared atomic proposition) in
+    order to reduce the number of atomic proposition, a source of
+    explonential explosion in several places of the translation
+    pipeline.  This relabel-bool optimization exists since Spot 2.4.
+    The new feature is that if, after relabel-bool, the formula still
+    has more than M atomic propositions, then spot::translate() now
+    attempts to relabel boolean subexpressions even if they have
+    overlapping atomic propositions, in an attempt to reduce the
+    number of atomic proposition even more.  Doing so has the slightly
+    unfortunate side effect of hindering some simplifications (because
+    the new atomic propositions hide their interactions), but it
+    usually incures a large speedup.  (See Issue #500, Issue #536.)
+
+    For instance on Alexandre's laptop, running
+      'ltlsynt --tlsf SPIReadManag.tlsf --aiger'
+    with Spot 2.11.6 used to produce an AIG circuit with 48 nodes in
+    36 seconds; it now produce an AIG circuit with 53 nodes in only
+    0.1 second.
+
 New in spot 2.11.6  (2023-08-01)
 
   Bug fixes: