pay, wallet: rename internal bolt11 vars to invstring.

And handle bolt12 strings if EXPERIMENTAL_FEATURES.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_plugin.py

@@ -387,6 +387,8 @@ def test_pay_plugin(node_factory):
     # Make sure usage messages are present.
     msg = 'pay bolt11 [msatoshi] [label] [riskfactor] [maxfeepercent] '\
           '[retry_for] [maxdelay] [exemptfee]'
+    if EXPERIMENTAL_FEATURES:
+        msg += ' [local_offer_id]'
     if DEVELOPER:
         msg += ' [use_shadow]'
     assert only_one(l1.rpc.help('pay')['help'])['command'] == msg