Make crate no_std-compatible

[alexpovel]

std Imports

Currently using this from std:

decompound/src/lib.rs

Line 18 in b2f4151

use std::{collections::BTreeSet, error::Error, fmt::Display};

• BTreeSet is in alloc (luckily: HashMap should be more widely usable rust-lang-nursery/portability-wg#11, see also nostd Sovereign-Labs/nmt-rs#16 for an example where this had to be circumvented; HashMap isn't relevant here though, as it breaks determinism), but it would be trivial to throw it out entirely and code around it; would make the code uglier though (manual deduplication... not a big deal but less pretty):

  decompound/src/lib.rs

  Lines 259 to 268 in b2f4151

  	let suffix_candidates = {
  	// Dedupe so no unnecessary work is done, but keep order for determinism
  	let mut set = BTreeSet::from_iter(vec![suffix.to_owned()]);
  	if options.contains(DecompositionOptions::TRY_TITLECASE_SUFFIX) {
  	let _ = set.insert(suffix.to_titlecase_lower_rest());
  	}
  	set
  	};

• Error in core is unstable; once that is stable, can move this crate to no_std. See: Tracking Issue for Error in core rust-lang/rust#103765

• Display is in core

3rd party dependencies

• decompound/Cargo.toml

  Line 17 in b2f4151

  bitflags = "2.4.0"

  is no_std as a feature:

  https://github.com/bitflags/bitflags/blob/472e392c0d082c0894b18fb31f4e68e0b145e29c/src/lib.rs#L241

• decompound/Cargo.toml

  Line 18 in b2f4151

  log = "0.4.19"

  is no_std as a feature:

  https://github.com/rust-lang/log/blob/b7ac2baea9f814feb1bf21fd05523840ff6a19c4/src/lib.rs#L328

• decompound/Cargo.toml

  Line 19 in b2f4151

  unicode_titlecase = "2.2.0"

  is no_std.