Extend hasher genericity to the remaining impls

[tb158]

Problem

#51 made Counter generic over its hasher, but four items were left in impl blocks written as
Counter<T, N>, which resolves to `Countsimply do not exist on a
counter built with any other hasher:

• Counter::into_map
• Counter::total
• impl FromIterator<(T, N)>
• impl SubAssign<I>

So swapping in a faster hasher silently c, counter.total(), counter.into_map(), and collecting from (item, count)` pairs, with nothing to suggest the
restriction is unintended.

Changes

Generalize the three impl blocks which ho

• into_map now returns HashMap<T, N, Ss own hasher rather than forcing RandomState`. For the default hasher the type is unchanged.
• total's type parameter is renamed froow names the hasher of the
  enclosing impl. Call sites are unaffected.
• FromIterator<(T, N)> gains S: BuildHe existing FromIterator` impl.
• SubAssign<I> gains S: BuildHasher, ` impl.

Compatibility

Non-breaking for annotated code, which ised
(collect::<Counter<_>>(), let c: Counter<char> = ...).

One inference caveat, the same one std accepts: Counter::from_iter(pairs) with no annotation
used to resolve S to RandomState becad now needs S pinned by
context. std made the same trade-off for
impl<K, V, S: Default + BuildHasher> Fro<K, V, S>.

Relationship to the constructor PR

Independent of #59 — either can merge firsts next to each other in
tests/tests.rs, so whichever merges second needs a trivial conflict resolution there (keep
both tests).

To keep that true, FromIterator<(T, N)>ruct literal rather than a
constructor: Counter::new() would return Counter<T, N, RandomState> once #59 lands, and
Counter::with_hasher does not exist untis the only form which
compiles either way. Once both are in, that body can be simplified to
Counter::with_hasher(S::default()) and p` import dropped — happy
to fold that into whichever branch merges second.

cargo test --all-features (18 unit + 19 integration + 47 doctests) and cargo clippy --all-features --all-targets are clean. r locally also builds and
passes (21 + 20 + 51).

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