Make HTLC_{SUCCESS,TIMEOUT}_TX_WEIGHT module private

[ksedgwic]

This is a follow-up to #1176

Hmm, two notes. (a) these are used pretty extensively in channel.rs too. There's a lot of churn on the code that stuff right now, so maybe don't update it here, but need to at least open an issue for that followup, (b) why are these now pub? Do y'all need these values downstream, and should we instead expose it as an inline getter that takes anchor as a paratmeter (and when we do so make them module-private, not even pub(super)).

[wpaulino]

Closing as this was done in 557a830.