feat: support no-alloc targets

[robjtede]

Closes #140

Summary

• Add an alloc feature, enabled by std and serde.
• Gate allocation-dependent formatting and parsing APIs behind alloc.
• Retain the existing no_std + alloc fixture and add a separate allocator-free fixture.
• Build both embedded configurations in CI.

Why

Users that only need ByteSize, constants, conversions, arithmetic, and Unit can now disable default features without linking an allocator.

Parsing keeps its existing String error type and remains behind alloc. A future breaking release can add allocation-free parsing errors without changing much except error types.

Validation

• just check
• just test
• just test-msrv
• just build-no-std
• just build-no-alloc

[nazar-pc]

This was a breaking change for those who had default-features = false since .display() would suddenly be missing after upgrade

[robjtede]

Fair enough. Yanked v2.5.0 and v2.6.0.

Usually adding new default features is okay but in this case you're right, it's not good that it removed an existing function in the process.

I'll get on cherry-picking the other parts of those releases soon.

[robjtede]

v2.7.0 released which reverts the alloc feature addition.