fix(echo): use built-in compression middleware instead of manual gzip/deflate

[BennyFranciscus]

What

Replaces hand-rolled compress/gzip and compress/flate writers in the /compression endpoint with Echo's built-in middleware.GzipWithConfig().

Why

Same pattern as #222 (Fiber) — the implementation rules require using framework-level APIs when available. Echo provides middleware.Gzip out of the box, so we should use it instead of hand-rolling compression.

Changes

• Register middleware.GzipWithConfig(middleware.GzipConfig{Level: 1}) globally
• Simplify /compression handler to just serve the JSON blob — the middleware handles encoding negotiation and compression automatically
• Remove unused compress/flate and compress/gzip imports

Note: Echo's built-in middleware only supports gzip (not deflate). Since gcannon sends Accept-Encoding: gzip, deflate and gzip takes priority, this shouldn't affect benchmark results. The middleware short-circuits on endpoints where no Accept-Encoding header is present, so overhead on other tests should be negligible.

Fixes #229

[BennyFranciscus]

Closing and reopening from my fork — branch protection prevents pushing the go.sum fix directly. New PR incoming.