make wasm32-* compatible

[DougAnderson444]

Current config does not allow for wasm32-* target builds.

This PR puts ssh-* behind a ssh feature flag, and only for non-wasm32 targets. uses ssh-key without default-features to enable wasm32 builds.

[target.'cfg(target_arch = "wasm32")'.dependencies]
ssh-key = { version = "0.6", default-features = false, features = [
  "alloc",
  "ecdsa",
  "ed25519",
] }

[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
ssh-key = { version = "0.6", features = ["alloc", "crypto", "ed25519"] }

Note that the crypto feature of ssh-key seems to still break wasm32 builds (I'm looking at you, p384), but there are many algs that aren't being used so this feature is paired down to only the 2 that are used.

[DougAnderson444]

This is blocking completion of cryptidtech/multikey#2

[DougAnderson444]

Improved PR:

• sets ssh-key default-features to false (for no std) instead of ssh feature flag
• needed to switch SSH Error to account for lack of std Error

This way, wasm32 targets get the SSH

[DougAnderson444]

@dhuseby this is ready for review now

[dhuseby]

LGTM