refactor(coin_selection)!: use Amount and SignedAmount for API and in…

…ternally

Using named types make the API and internal code easier to read and reason
about since it makes it clear that the values are bitcoin amounts. Also to
create these types the units (ie .from_sat()) must be specified.

Using Amount and SignedAmount also makes internal code safer against overflow
errors. In particular because these types will panic if an amount overflow
occurs. Using u64/i64 on the otherhand can silently rollover. See:
https://doc.rust-lang.org/book/ch03-02-data-types.html#integer-overflow