bech32: another around 10% speedup for bech32_decode

turns out stdlib ord() is somewhat slow;
also, only lookup data chars once

benchmarked with:
```
import time
import electrum
from electrum.segwit_addr import bech32_decode

electrum.constants.set_testnet()

inv = "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"

def f():
  for _ in range(10000):
    addr = bech32_decode(inv, ignore_long_length=True)

t0 = time.time()
f()
t1 = time.time()
print(f"{t1-t0:.4f}")
```

electrum/segwit_addr.py

@@ -61,19 +61,23 @@ def bech32_encode(hrp, data):
     return hrp + '1' + ''.join([CHARSET[d] for d in combined])
 
 
-def bech32_decode(bech, ignore_long_length=False):
+def bech32_decode(bech: str, ignore_long_length=False):
     """Validate a Bech32 string, and determine HRP and data."""
-    if ((any(ord(x) < 33 or ord(x) > 126 for x in bech)) or
-            (bech.lower() != bech and bech.upper() != bech)):
+    bech_lower = bech.lower()
+    if bech_lower != bech and bech.upper() != bech:
         return (None, None)
-    bech = bech.lower()
     pos = bech.rfind('1')
     if pos < 1 or pos + 7 > len(bech) or (not ignore_long_length and len(bech) > 90):
         return (None, None)
-    if not all(x in CHARSET for x in bech[pos+1:]):
+    # check that HRP only consists of sane ASCII chars
+    if any(ord(x) < 33 or ord(x) > 126 for x in bech[:pos+1]):
         return (None, None)
+    bech = bech_lower
     hrp = bech[:pos]
+    try:
         data = [_CHARSET_INVERSE[x] for x in bech[pos+1:]]
+    except KeyError:
+        return (None, None)
     if not bech32_verify_checksum(hrp, data):
         return (None, None)
     return (hrp, data[:-6])