Fix Complex error size

[mkeeter]

Idol calls returning a Complex error type return a non-zero error code of 1, but also serialize their error into the buffer provided by the caller. Right now, we're not necessarily sizing that buffer correctly when making HIF and RPC calls! If the ok type is smaller than the error type, then the buffer will be undersized and bytes will be dropped on the floor during the sys_reply call.

sys_reply does not report errors, but it expects the server to check the buffer size itself. A separate issue in Idol (oxidecomputer/idolatry#36) means that the generated server code makes the same mistake in checking the buffer size; otherwise, we would have seen this error much sooner!

(Luckily, this all works correctly in task-to-task calls, because Idol's generated client API correctly computes the maximum REPLY_SIZE and passes correctly-sized buffers)

This PR correctly computes the reply_size() for Idol operations using Complex error types. It also fixes an error in computing serialized size for enum types (we were taking the sum of variant sizes instead of the max, which is wrong).