Soften Requirement on std Since Error Moved from std to core

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For context, Rust has 2 "levels" of standard library:

• core: the 'true' standard library which is always available no matter what.
• std: a superset which includes extra things only available on non-embedded systems

The slice-codec has a "feature flag" named std which is enabled by default, but if it's disabled, our crate supports being used without std: on embedded systems.

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Rust has a general trait Error, which all error like types should implement.
But, for years, this trait was std::error::Error: only available if you were using std.
In Rust 1.81 (released 1.5 years ago) they moved it to core::error::Error, so now it's part of core.

This PR implements #716: we remove this feature gate where it is no longer necessary.

[Copilot]

Pull request overview

This PR updates the codebase to take advantage of the core::error::Error trait that was stabilized in Rust 1.81, removing the previous requirement on the std feature for error handling. The changes replace references to std::error::Error with core::error::Error and change feature gates from std to alloc where appropriate.

Changes:

• Replaced all references to std::error::Error with core::error::Error
• Changed feature gates from std to alloc for error source tracking functionality
• Simplified conditional compilation in buffer handling code

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
slice-codec/src/error.rs	Updated Error type to use core::error::Error instead of std::error::Error, changed feature gates from std to alloc for source error tracking
slice-codec/src/buffer/vec.rs	Removed conditional compilation that distinguished between std and non-std error handling, now consistently uses new_with_source

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