Currently the `serde` feature requires an allocator, this is a hang over
from code being written for the `bitcoin` crate.

This was not found because we are not testing `primitives` correctly in
CI (there is the `fuzz` bug and also `primitives` is not even in the
CRATES list).

This should have been done when we introduced the `primitives`
crate - epic fail.

Intentionally put it before `fuzz` because of the known bug in the CI
script where nothing after `fuzz` gets tested.

The `hash_reader` function is new and unreleased, it should never have
been put into the `sha256t_hash_newtype` macro, and its broken.

The `hash_reader` function is only available when `bitcoin-io` is
enabled - it should be feature gated.

We lost this impl at some stage in the last few weeks and did not notice
because of a bug in `run_task` [0].

Implement `JsonSchema` for `siphash` as we do for all the other hash
types.

[0] rust-bitcoin/rust-bitcoin-maintainer-tools#10

In rust-bitcoin#3010 we added a `length` field to the `sha256::Midstate` which broke
the `schemars` test but we did not notice because of the current bug [0]
in the `run_task` CI script.

[0] rust-bitcoin/rust-bitcoin-maintainer-tools#10

Note:

- The `extra_test.sh` script runs for all toolchains.
- The `schemars` crate does not use the repo lock files.
- We need to pin some deps when building the `schemars` test.

Pin in the `extra_test.sh` script, and mention it in the docs so the
docs don't go stale next MSRV update.

This was previously unnoticed because of the `run_task` bug.

ref: rust-bitcoin/rust-bitcoin-maintainer-tools#10

In recent work making functions on hash types inherent it seems we
missed `siphash`. Add inherent getters/setters to the `siphash::Hash`
type and call through to them from the `Hash` trait impl.

The `serde` impls and `FromStr` are missing from `siphash`, add them.

There is a bug in `run_task` [0] that drastically reduces our test
coverage. We can bypass it by putting `fuzz` last in the list.

Note, we re-order all crates before `fuzz` to be in alphabetic order,
this is required because when adding `primitives` we poked it in
before `fuzz` to make sure it ran.

[0] rust-bitcoin/rust-bitcoin-maintainer-tools#10