Optimize the vis_heap_chunks by creating VALID_CHARS once

This commit optimizes the `bin_ascii` function used by the `vis_heap_chunks` command.

That function executed the following line on each call:
```
    valid_chars = list(map(ord, set(printable) - set("\t\r\n\x0c\x0b")))
```

And it could be called thousand times, e.g. 90k on a benchmark.

This commit moves the creation of the `valid_chars` list to the global space so it is computed only once.

As a result, on a simple benchmark we improved the speed of `vis_heap_chunks` command from 4.6s to 3s.

pwndbg/commands/heap.py

@@ -1,5 +1,6 @@
 import argparse
 import ctypes
+from string import printable
 from typing import Dict
 from typing import List
 
@@ -973,11 +974,11 @@ def vis_heap_chunks(
         )
 
 
-def bin_ascii(bs):
-    from string import printable
+VALID_CHARS = list(map(ord, set(printable) - set("\t\r\n\x0c\x0b")))
+
 
-    valid_chars = list(map(ord, set(printable) - set("\t\r\n\x0c\x0b")))
-    return "".join(chr(c) if c in valid_chars else "." for c in bs)
+def bin_ascii(bs):
+    return "".join(chr(c) if c in VALID_CHARS else "." for c in bs)
 
 
 def bin_labels_mapping(collections):