Proposal: Providing a no-runtime-rng version using a feature gate

[sjfhsjfh]

Currently, ahash and getrandom is used

ahash: default-features = false, features = ["std"]
getrandom: rewrite a small rng using rand = { version = "0.9.0", default-features = false, features = ["small_rng"] }

These optional modifications can be applied when the runtime-rng feature flag is not present.

I made a working example here, but I'm not very sure if this is the best way of doing this and there're some problems(e.g. the image feature uses rand crate, this can be seen from cargo tree --target all -i getrandom), so it is currently just a proposal.

[sjfhsjfh]

I drafted a PR #276 and please comment if any further info should be provided

[MathNya]

@sjfhsjfh
Thank you for your suggestion.
I understand that this suggestion will benefit when executed in WebAssembly.
(Sorry if I'm wrong, I'm not sure if this is the case).

Regarding the issue of image using rand, I only use image to measure the size of the image to begin with.
I found imagesize which has more limited functionality than image.
This may be more appropriate.
I will consider replacing it.