wire: add towire_int() and use it in connectd

Add towire_int() and fromwire_int() functions to "(de)serialize"
"int". This will only work as long as both the caller of towire_int()
and the caller of fromwire_int() use the same in-memory representation
of signed integers and have the same sizeof(int).

Changelog-None

tools/generate-wire.py

@@ -185,6 +185,7 @@ class Type(FieldSet):
         'bool',
         'amount_sat',
         'amount_msat',
+        'int',
         'bigsize',
         'varint'
     ]
@@ -199,6 +200,7 @@ class Type(FieldSet):
         'secp256k1_ecdsa_recoverable_signature',
         'wirestring',
         'double',
+        'int',
         'bigsize',
         'varint',
     ]