Make imap-types no_std compatible

[soywod]

With this PR, imap-types is now #![no_std] (requires alloc). This is non-breaking: features, defaults and the public API are unchanged. imap-codec is left for a follow-up PR (motivation: pimalaya/io-imap#4).

Changes

• std:: paths became core::/alloc::, and items that came from the prelude are now imported explicitly.
• extern crate std is gated on what brings std:
  • arbitrary: the derive expands to ::std::thread_local!.
  • tag_generator: uses rand::rng().
  • test: some tests use println!.
• thiserror and serde use default-features = false. thiserror then implements core::error::Error, which is the same trait as std::error::Error on
  MSRV 1.85.
• SequenceSet::normalize uses a BTreeSet instead of a HashSet. The set is always drained before the final loop, so output is unchanged.

Why no std feature

A default std feature that gates TagGenerator::generate would break users who combine default-features = false with tag_generator, as imap-codec
does. Keeping tag_generator std-only gives no_std to everyone else for free.

Verification

• Builds for thumbv7em-none-eabihf with every feature except arbitrary and tag_generator.
• clippy and tests are clean across all feature combinations.
• imap-codec tests pass.

Follow-ups

• imap-codec: EncodeContext implements std::io::Write without doing real I/O.
• A no_std CI build job.

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Refs: #495

[coveralls]

Coverage Report for CI Build 36941102953

Coverage remained the same at 91.697%

Details

• Coverage remained the same as the base build.
• Patch coverage: 10 uncovered changes across 7 files (25 of 35 lines covered, 71.43%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
imap-types/src/extensions/binary.rs	2	0	0.0%
imap-types/src/extensions/condstore_qresync.rs	2	0	0.0%
imap-types/src/response.rs	3	1	33.33%
imap-types/src/core.rs	10	9	90.0%
imap-types/src/extensions/sort.rs	1	0	0.0%
imap-types/src/extensions/thread.rs	3	2	66.67%
imap-types/src/extensions/utf8.rs	2	1	50.0%
Total (17 files)	35	25	71.43%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	11899
Covered Lines:	10911
Line Coverage:	91.7%
Coverage Strength:	726.78 hits per line

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💛 - Coveralls

[duesee]

Looks good to me, thank you very much!