In preparation for enabling `rustfmt` crate wide add various skip
statements and do minor format fixes.

Note, for some reason adding `rustfmt::skip` to the
`sha256t_hash_newtype` causes build to fail due to ambiguous naming (we
have the exact same macro in `bitcoin_hashes`). The macros are identical
except for the `rust-bitcoin` version adds docs to the created structs.
Until that can be pushed upstream to `bitcoin_hashes` I've renamed the
macro to `_sha256t_hash_newtype` and added a comment.

Add a `rustfmt` configuration file setting a bunch of configuration
options. All options that are not the default value include a comment
with the default value.

Run `cargo +nightly fmt`. No other changes made.

In order to make rustfmt leave the comment on the same line as the magic
number we have to put it _before_ the number.

In order to get the rustfmt to play nicely with multiline math
statements that include comments put the operator at the start of the
line.