Move invoice utilities to `ChannelManager`

[TheBlueMatt]

I thought I already opened an issue for this, but apparently not. Now that we swapped the dependency order between lightning-invoice and lightning we can also move the crap we shoved in lightning::ln::bolt11_payment into ChannelManager (possibly leaving the code in a separate file), making paying BOLT 11 invoices the same first-class citizen as paying BOLT 12 invoices. We might also consider moving ln::invoice_utils into some kind of builder pattern (or something where we pass a configuration object to ChannelManager and it spits out a BOLT 11) like we have for BOLT 12.