Make Message a regular class instead of a Struct

[moberegger]

This is a weird one that I hope isn't too contentious. The initialization of Message was coming up as a CPU hotspot in our production profiles. I didn't think anything of it and figured it was just because it was called so often. Then I noticed it was a Struct and got curious.

I compared the initialization between a regular class and a Struct, and there is a noticeable improvement when using a plain old Ruby class - at least on Ruby 4 with yjit on.

ruby 4.0.2 (2026-03-17 revision d3da9fec82) +YJIT +PRISM [arm64-darwin25]
Warming up --------------------------------------
Message.new (Struct)   879.474k i/100ms
 Message.new (Class)     1.480M i/100ms
Calculating -------------------------------------
Message.new (Struct)     11.474M (± 6.6%) i/s   (87.15 ns/i) -     57.166M in   5.011466s
 Message.new (Class)     18.826M (± 6.2%) i/s   (53.12 ns/i) -     94.715M in   5.056730s

Comparison:
Message.new (Struct): 11473914.4 i/s
 Message.new (Class): 18826156.1 i/s - 1.64x  faster

This PR refactors Message to a regular Ruby class. Included is an implementation of == for use by tests.