feat(bitfield): add Pair::pack_from_{src, dst} (#294)

This commit adds `pack_from_src` and `pack_from_dst` const fn methods to
the bitpacking `Pair{N}` types in `mycelium_bitfield`. It also changes
the `dst` type for `Pack{N}::pair_with` from `&Self` to `Self`, since
packing specs are `Copy`able.

bitfield/src/pack.rs

@@ -286,7 +286,7 @@ macro_rules! make_packers {
                 const fn mk_mask(n: u32) -> $Bits {
                     if n == 0 {
                         return 0
-                    };
+                    }
                     let one: $Bits = 1; // lolmacros
                     let shift = one.wrapping_shl(n - 1);
                     shift | (shift.saturating_sub(1))
@@ -591,7 +591,7 @@ macro_rules! make_packers {
                 /// into another location, and vice versa.
                 pub const fn pair_at(&self, at: u32) -> $Pair {
                     let dst = $Pack::starting_at(at, self.bits());
-                    self.pair_with(&dst.typed())
+                    self.pair_with(dst.typed())
                 }
 
                 /// Returns a pair type for packing bits from the range
@@ -606,7 +606,7 @@ macro_rules! make_packers {
                 /// # Note
                 /// The two ranges must be the same size. This can be asserted
                 /// by the `assert_valid` method on the returned pair type.
-                pub const fn pair_with(&self, dst: &Self) -> $Pair {
+                pub const fn pair_with(&self, dst: Self) -> $Pair {
                     // TODO(eliza): validate that `dst.shift + self.bits() < N_BITS` in
                     // const fn somehow lol
                     let (dst_shl, dst_shr) = if dst.shift > self.shift {
@@ -803,6 +803,25 @@ macro_rules! make_packers {
                     Self(packer.pack_truncating(value, self.0))
                 }
 
+                /// Pack bits from `src` into `self`, using the packing pair
+                /// specified by `pair`, with `self` serving as the "destination" member
+                /// of the pair, and `src` serving as the "source" member of the
+                /// pair.
+                #[inline]
+                pub const fn pack_from_src(self, value: $Bits, pair: &$Pair) -> Self {
+                    Self(pair.pack_from_src(self.0, value))
+                }
+
+                /// Pack bits from `dst` into `self`, using the packing pair
+                /// specified by `pair`, with `self` serving as the "siyrce" member
+                /// of the pair, and `dst` serving as the "destination" member of the
+                /// pair.
+                #[inline]
+                pub const fn pack_from_dst(self, value: $Bits, pair: &$Pair) -> Self {
+                    Self(pair.pack_from_dst(value, self.0))
+                }
+
+
                 /// Pack bits from `value` into `self`, using the range
                 /// specified by `packer`.
                 ///