tx-firehose: prefer where over let for local bindings

Sweep the tx-firehose sources to use where clauses instead of
let ... in wherever it's in scope. Only the let bindings that
close over IO-bound values or lambda-captured variables (where a
where clause can't reach) are kept as let.

Also: switch feeLovelace's local pattern-let to a where-level
pattern binding on the Coin newtype for the same reason.

Author: ch1bo