Add specs for all of IO::Buffer constructors

[trinistr]

Continuing from #1297

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More weird stuff:

• .map:
  • Specifying non-0 offset guarantees that everything breaks: size is computed incorrectly, crashing VM on reading the whole buffer, on macOS even a bounded read crashes VM, on Windows valid sizes are ???
    • https://bugs.ruby-lang.org/issues/21700
    • Fix multiple bugs in IO::Buffer.map and update its documentation ruby#15264
• .new:
  • Passing LOCKED flag is intended? There is a test for that. But this seems like a bad idea to me, as it's not freeable (GC can still free it, though, I think).

[eregon]

@ioquatix Could you review this maybe?

[trinistr]

@ioquatix Could you review this maybe?

This is still a very early draft, but that offset parameter made me create an issue and a PR to ruby directly, so, yeah 🥹 Reviewing the PR with fixes for the method is probably more useful currently.