[RFC] Orthogonality between signed/unsigned and Fixed/Normed

[kimikage]

As commented in #143 (comment), Fixed/Normed (i.e. the difference in scaling factor) and signed/unsigned are orthogonal concepts.

Type (scaling)	Fixed (2^f)	Normed (2^f-1)
signed	✔️	❌ N/A
unsigned	❌ N/A	✔️

Historically, Fixed is signed and Normed is unsigned, but I think we should separate the two orthogonal concepts as much as possible.

I do not intend to support unsigned Fixed and signed Normed right now. However, I think that it is "safe" to describe in README.md that they may be supported in the future, because we have no systematic way to notify the downstream packages of the breaking changes to the existing Fixed and Normed.
(Whether relaxing is considered "safe" or "unsafe" depends on the point of view.)

At least I think it is worth writing that in CONTRIBUTING.md. It clarifies whether the code should be written in "src/FixedPointNumbers.jl", "fixed.jl" or "normed.jl". I think that the code which is not related to scaling factors should be written in "src/FixedPointNumbers.jl", i.e. should be commonized.