Converge Opal's stdlib/buffer and Ruby 3.1 IO::Buffer

[elia]

We have a stdlib/buffer library and ruby 3.1 introduced IO::Buffer. They're similar enough that we can consider just keeping the latter. Deprecating it once a replacement IO::Buffer implementation is available and removing stdlib/buffer in a following major version.

Additional context

See "Buffer, Buffer::View, Buffer::Array" in #2231
https://rubyapi.org/3.1/o/io/buffer
https://github.com/opal/opal/blob/master/stdlib/buffer.rb

[hmdne]

Won't be finalized for v1.5. Pushing it to v1.6.

[hmdne]

Moving to v1.7 (or v2.0)