argon2: more const-ness

[C0D3-M4513R]

Redo of #438 after #440.
Rebased original PR, then applied fixes and then sqashed

[C0D3-M4513R]

With both above approaches out of the question I see no other alternative than to make a breaking change here and wait for a merge with the next breaking change.

Also I don't necessarily think that opening an issue to go alongside this is a good idea.
Why have two places to discuss things, rather than one?

[tarcieri]

I think this PR could've benefitted from some up-front design discussion.

I'm not sure it necessarily makes sense to constify the builder. And doing so, at least the way you propose, means copying the entire thing every time you use any builder method. There are some tradeoffs there I think are worth discussing.

[C0D3-M4513R]

Well I don't want to duplicate any code from the Builder's build method in the Param's new method.
And with the state of const fn as they are right now, I do not see a way of doing so without either of the ways I proposed, which would be:

• adding a new new method to the builder that works in const contexts
• Making the Builder const compatible

Since I do not see us coming on a common factor I will just keep using my fork of password-hashes argon2 for now until something is implemented.

Also the change 6e9662a would not copy the Builder, it moves it which is something different.
Also in practice it should make no difference, since I am returning the same builder anyways.
So the variable the builder is assigned to does not actually change.

[tarcieri]

Well I don't want to duplicate any code from the Builder's build method in the Param's new method.

Okay, so it seems like part of the problem for now is Params::new is using the builder internally in its current implementation.

But Params::new is much more ideal for constification since it already accepts all of the parameters explicitly.

Perhaps instead of trying to constify the Builder, you could eliminate the internal use of it within Params and make const fn new which could then in turn be used by the Builder, rather than the other way around.

[C0D3-M4513R]

Changed as requested.

[tarcieri]

Thank you!