Change coin_selection and DescriptorExt::dust_value to use Amount type

[notmandatory]

Description

refactor(coin_selection)!: use Amount and SignedAmount for API and internally
refactor(chain)!: use Amount for DescriptorExt::dust_value()

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.

Notes to the reviewers

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

This is a continuation of the work done in #1595. Since this is an API breaking change I would like to include it in the 1.0.0-beta milestone if possible.

Changelog notice

• Change coin_selection to use Amount instead of u64 for all bitcoin amounts.
• Change DescriptorExt::dust_value() to return an Amount instead of u64.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

Thanks to @thesimplekid for bringing to my attention that native rust integer types can silently roll-over. Even though this is very unlikely to happen, using Amount/SignedAmount in coin_selection is a good way to make sure it never happens.

The rest of the code base is already updated to use Amount in APIs and internally.

[notmandatory]

Just as an FYI, I did a little test to confirm that rust integer underflow does wrap and isn't caught as a static check if the values aren't known at compile time. This only applies when built in release mode, in debug mode it panics.

#[test]
#[cfg(not(debug_assertions))]
fn test_integer_underflow() {
    use rand::Rng;
    let num = rand::thread_rng().gen_range(11..21);
    let ten: u64 = 10;
    let underflow = ten - num;
    // println!("10 - {} => underflow {}", num, underflow);
    assert!(underflow > 0)
}

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