units: Make fee module public

The `fee` module is now empty as far as API surface. It still holds the
`core::ops` impls for fee calculation. It also does have useful
rustdocs which are not currently visible online because module is
private.

Make the module public. Functionally all this does is provide a place
for the module level docs under a discoverable name.

Improve the docs by adding a bunch of links to fee related functions.

Author: tcharding

units/src/fee.rs

@@ -10,6 +10,18 @@
 //!
 //! We provide `fee.checked_div_by_weight_ceil(weight)` to calculate a minimum threshold fee rate
 //! required to pay at least `fee` for transaction with `weight`.
+//!
+//! We support various `core::ops` traits all of which return [`NumOpResult<T>`].
+//!
+//! For specific methods see:
+//!
+//! * [`Amount::checked_div_by_weight_floor`]
+//! * [`Amount::checked_div_by_weight_ceil`]
+//! * [`Amount::checked_div_by_fee_rate_floor`]
+//! * [`Amount::checked_div_by_fee_rate_ceil`]
+//! * [`Weight::checked_mul_by_fee_rate`]
+//! * [`FeeRate::checked_mul_by_weight`]
+//! * [`FeeRate::to_fee`]
 
 use core::ops;
 

units/src/lib.rs

@@ -34,7 +34,6 @@ extern crate alloc;
 #[cfg(feature = "std")]
 extern crate std;
 
-mod fee;
 mod internal_macros;
 mod result;
 
@@ -48,6 +47,7 @@ pub mod _export {
 
 pub mod amount;
 pub mod block;
+pub mod fee;
 pub mod fee_rate;
 pub mod locktime;
 pub mod parse;