In preparation for removing a bunch of macros move all the modules to
`<mod>/mod.rs`.
Do so by running the following shell:
```bash
for mod in hash160 ripemd160 sha1 sha256 sha256d sha256t \
sha384 sha512 sha512_256 siphash24 hkdf hmac; do
mkdir $mod
mv "$mod.rs" "$mod/mod.rs"
done
```
Internal change only.