The Rust API guidelines state that macros should be evocative of the
output, which is a sensible recommendation. We already had this for
`hash_newtype!` macro but didn't for sha256t version.
This changes the macro to have this syntax:
```rust
sha256t_hash_newtype! {
// Order of these structs is fixed.
/// Optional documentation details here. Summary is auto-generated.
/*pub*/ struct Tag = raw(MIDSTATE_BYTES, LEN);
/// Documentation here
#[hash_newtype(forward)] // optional, default is backward
/*pub*/ struct HashType(/* attributes allowed here */ _);
}
```
Closes#1427
095b7958dd