Add specs for all of IO::Buffer constructors

[trinistr]

Continuing from #1297

• Added a spec for .map. It has a lot of edge cases.
  • In my opinion, basically all cases which result in a syscall error are undefined behavior, but most of them at least can be specified. Concrete errors are not specified due to differences between systems.
  • Offset and size were not really checked before Fix multiple bugs in IO::Buffer.map and update its documentation ruby#15264 (which should land in Ruby 4.0).
• Added specs for .for and .string. Not much to say about these, they behave according to documentation and don't have complicated arguments.
• Added more cases for #initialize (.new).
• Removed most cases for predicate methods, leaving just a few representative examples, as flags are checked in constructor specs.

---

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
  • size can be larger than file's size, which may somewhat work on some systems or cause crashes on access.
    • Fix multiple bugs in IO::Buffer.map and update its documentation ruby#15264 prohibits this.
• .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.