Mirror the whitespacing in `rust-bitcoin` by doing:

- Only use single line of whitespace between sections
- Separate optional dependencies from non-optional ones

Currently we use default features for the `bitcoin_hashes` dependency,
doing so breaks the `no-std` feature in `rust-bitcoin` because `std` is
part of `bitcoin_hashes` default feature set.

Disable `bitcoin_hashes` default features, no changes to `rust-bitcoin`
are require after this change since we manually turn on `std` and
`alloc` as part of the `std`/`no-std` features of `rust-bitcoin`.

For other users of `rust-secp256k1` this is a breaking change but is
unlikely to cause too much bother because `std` is so commonly used.

Currently we use 'no default features' for the `bitcoin_hashes`
dependency. Doing so means that if users want the `std` feature they
need to explicitly add a `bitcoin_hashes` dependency even though we
re-export `bitcoin_hashes` as `hashes`. This means that in the common
case the re-export is pointless. As an example, `rust-bitcoin`
unnecessarily requires an explicit dependency on `bitcoin_hashes`.

Add `bitcoin-hashes-std` feature so that users do not need an explicit
dependency in the common use case.

Change the test matrix to only test '*-std' features when 'std' is
enabled since enabling one without the other is illogical. Please note,
this replaces the test run of feature 'std'+'rand'+'rand-std' with just
'std'+'rand-std' because enabling 'rand-std' enables 'rand' so the
explicit additional feature is redundant.