Use collections from the alloc crate in percent_encoding

[grego]

Use collections from alloc in the percent_encoding crate, thus making it fully no_std compatible.

Additionally implements From<PercendEncode>> and From<PercendDecode>> for beef::Cow, using an optional beef feature gate.
Edit: ditched the beef implementation as it didn't pass all of the checks.

[djc]

Some downstream users might not have alloc available, right? I think this makes their lives worse?

[grego]

Fair enough. Added all the usage of alloc types behind an optional alloc feature gate.

[djc]

LGTM, thanks!

[grego]

Could you, please, publish a version with this PR? I need it for a project.

[tesaguri]

Technically speaking, this is a breaking change if a downstream user specifies default-features = false, though I don't think it would be a problem in practice since percent-encoding has had no feature flags before. I just thought it worth mentioning.

[djc]

I wouldn't be surprised if there are people using default-features = false anyway out of habit.

[tesaguri]

I wouldn't be surprised if there are people using default-features = false anyway out of habit.

I've done a quick search for packages in rust-lang/crates.io-index whose most recently published version has dependency on percent-encoding with default features disabled.

$ curl -fL -o crates.io-index.zip https://github.com/rust-lang/crates.io-index/archive/refs/heads/master.zip
$ stat -f '%Sm' -t '%FT%T%z' crates.io-index.zip
2021-12-08T17:43:38+0900
$ unzip crates.io-index.zip
$ find crates.io-index-master -type f ! -name config.json | xargs -n 253 tail -q -n 1 | jq -c 'select(.deps | any([.name == "percent-encoding", .default_features == false] | all)) | {name, vers}'

Result:

{"name":"instagram-web-api","vers":"0.1.1"}
{"name":"twilight-lavalink","vers":"0.8.0"}
{"name":"twilight-http","vers":"0.8.0"}
{"name":"mendes","vers":"0.0.60"}
{"name":"atlassian-app-auth","vers":"1.0.1"}
{"name":"gcs-rsync","vers":"0.1.10"}
{"name":"cloud-storage-rs","vers":"0.4.3"}
{"name":"cloud-storage","vers":"0.11.0-rc.1"}

So there are indeed downstream crates with default-features = false.

I didn't build them with this change applied so I don't know if this breaks the above crates (I'm too nervous to build random packages in an unsandboxed environment and too lazy to set up a Crater-like sandboxed environment or manually review each package).