[RFC] Reserving aliases for unsigned `Fixed` and signed `Normed`

As noted in #199, unsigned `Fixed` and signed `Normed` are mathematically definable, but we still have some preparations to implement them.

Although those types are certainly not used often, but in fact, I occasionally use unsigned `Fixed` in private projects. In that case, I give those types redundant names.

However, when we handle concrete `Fixed`/`Normed` numbers, we often use the aliases (e.g., `Q0f7`, `N0f8`). If we have the predetermined aliases for unsigned `Fixed` and signed `Normed`, they can be used to make the future migration easier.

See also #51

## Candidates
### unsigned `Fixed`
- `UQxfy` (*U*nsigned *Q*)
- `Qxfy` (`x+y == 8sizeof(T)`)
- `Uxfy` (*U*nsigned)
- `Rxfy` (following `Q`)
- `Fxfy` (*F*ixed)
- `Xxfy` (fi*X*ed)

### signed `Normed`
- `SNxfy` (*S*igned *N*)
- `Nxfy` (`1+x+y == 8sizeof(T)`)
- `Sxfy` (*S*igned. cf. #143)
- `Mxfy` (nor*M*ed. followed by `N`)

Renaming `Nxfy` to `UNxfy` is a consistent practice, but it defeats the purpose of making the migration easier.