Simplify the interface around request buffer

[ubnt-intrepid]

Fixes #261.

[Copilot]

Pull request overview

This PR simplifies the request buffer interface by consolidating the previously separate buffer types (SpliceBuf and FallbackBuf) into a unified RequestBuf type. The refactoring addresses issue #261 by removing generic trait-based abstractions and replacing them with a concrete implementation that internally handles both pipe-based (splice) and vector-based buffer strategies.

• Unified buffer types: SpliceBuf and FallbackBuf are now internal variants of RequestBuf
• Simplified API: Removed TryReceive and ToParts traits in favor of concrete methods on RequestBuf
• Streamlined initialization: Replaced separate new_splice_buffer() and new_fallback_buffer() methods with a single new_request_buffer() that automatically selects the appropriate implementation

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/buf.rs	Consolidates buffer types into RequestBuf with internal BufKind enum; removes trait abstractions; adds RemainingData enum for uniform handling of remaining data
src/session.rs	Updates API to use RequestBuf and removes generic type parameters; replaces separate buffer creation methods with unified new_request_buffer()
src/connect.rs	Updates to use new RequestBuf::new_vec() constructor during connection initialization
examples/poll.rs	Updates to use new_request_buffer() instead of new_splice_buffer()
examples/hello.rs	Updates to use new_request_buffer() instead of new_splice_buffer()
examples/heartbeat_entry.rs	Updates to use new_request_buffer() instead of new_splice_buffer()
examples/heartbeat.rs	Updates to use new_request_buffer() and adds mut to remains binding for io::Read usage
examples/basic.rs	Updates to use new_request_buffer() instead of new_fallback_buffer() and handles Result type