Reuse underlying array if RawMessage has sufficient capacity

[zensh]

Description (motivation)

This is a performance optimization for implementing cbor-patch.

PR Was Proposed and Welcomed in Currently Open Issue

This PR was proposed and welcomed by maintainer(s) in issue #

Closes #

Checklist (for code PR only, ignore for docs PR)

• Include unit tests that cover the new code
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[fxamacker]

@zensh Thanks for the PR! Looks great!