Add upfront fees, as parsed out of extra data TLVs, to update_add_htlc.
An aliased type is used because it will later be important to be able
to distinguish between a zero value and the absence of the field.

This commit adds the unconditional fees that are optionally included in
UpdateAddHtlc's extra data TLVs to our payment descriptor struct and
accounts for them in balance calculations for calculating our new commit
view.

Refactor our payload creation to perform tlv parsing and validation
in separate steps. This will become useful when we need to compare
fields in our onion payload with the fields that were transmitted
to us in update_add_htlc (rather than needing to pass the incoming
htlc all the way down to parsing, which is a clumsy layering
violation).

TODO: I haven't run through this properly, need to check each state
update in more detail!