global: expose all fmt_X functions for direct use, make uniform.

We have various functions to convert to a string, rename them all so we can
count on fmt_X being the formatter for struct X, and make them all return
`char *`.

Sometimes they existed but were private, sometimes they had a
different name.  Most take a pointer, but simple types pass by copy:
short_channel_id, amount_msat and amount_sat.

The following public functions changed:
1. psbt_to_b64 -> fmt_wally_psbt.
2. pubkey_to_hexstr -> fmt_pubkey.
3. short_channel_id_to_str -> fmt_short_channel_id (scid by copy now!)
4. fmt_signature -> fmt_secp256k1_ecdsa_signature
5. fmt_amount_sat/fmt_amount_msat pass copy not pointer, return non-const char *.
6. node_id_to_hexstr -> fmt_node_id

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

plugins/sql.c

@@ -768,7 +768,7 @@ static void delete_channel_from_db(struct command *cmd,
 			   tal_fmt(tmpctx,
 				   "DELETE FROM channels"
 				   " WHERE short_channel_id = '%s'",
-				   short_channel_id_to_str(tmpctx, &scid)),
+				   fmt_short_channel_id(tmpctx, scid)),
 			   NULL, NULL, &errmsg);
 	if (err != SQLITE_OK)
 		plugin_err(cmd->plugin, "Could not delete from channels: %s",
@@ -904,7 +904,7 @@ static void delete_node_from_db(struct command *cmd,
 			   tal_fmt(tmpctx,
 				   "DELETE FROM nodes"
 				   " WHERE nodeid = X'%s'",
-				   node_id_to_hexstr(tmpctx, id)),
+				   fmt_node_id(tmpctx, id)),
 			   NULL, NULL, &errmsg);
 	if (err != SQLITE_OK)
 		plugin_err(cmd->plugin, "Could not delete from nodes: %s",