Only create mmap once per binary

We keep the context manager in play just to avoid refactoring all
of the references to that.

src/smgdc/validate.py

@@ -52,6 +52,7 @@ class BaseBinary:
     path: pathlib.Path
     angr: angr.Project
     _file: io.IOBase
+    _mm: mmap.mmap
 
     def __init__(self, path: pathlib.Path, cache_path: pathlib.Path | None = None):
         self.path = path
@@ -64,22 +65,15 @@ class BaseBinary:
             cached_proj.write_bytes(pickle.dumps(self.angr))
         else:
             self.angr = pickle.loads(cached_proj.read_bytes())
+        self._mm = mmap.mmap(self._file.fileno(), 0, access=mmap.ACCESS_READ)
 
     @contextlib.contextmanager
     def mmap(self):
-        mm = mmap.mmap(self._file.fileno(), 0, access=mmap.ACCESS_READ)
-        yield mm
-
-        # FIXME: We currently don't close this because an error gets thrown at
-        #        ``LocationEntry.calculate_phys_address()`` due to exported pointers.  It's not
-        #        clear to me if the issue stems from ``ByteSignature.expr.finditer()`` or
-        #        something else at this point.
-        # mm.close()
+        yield self._mm
 
     def read(self, address, size) -> bytes:
         # shorthand to read a value from a physical file
-        with self.mmap() as memory:
-            return memory[address : address + size]
+        return self._mm[address : address + size]
 
 
 class WindowsBinary(BaseBinary):