Update fiber scheduler to support Ruby 4.1

[rsamoilov]

• io_write - V4 uses the new synchronous Iodine::Scheduler.write; V3 preserves the existing behaviour by using Iodine::Scheduler.write_async (it was renamed from write)
• io_read - both V3 and V4 use Iodine::Scheduler.read that has slightly different behaviour based on the scheduler version; to support the new single-operation semantics, the EINTR check was removed from Iodine::Scheduler.read and added into V3's io_read
• both io_read and io_write now pass buffer to Iodine - Iodine operates on the buffer directly to avoid unnecessary string allocations with IO::Buffer#get_string and IO::Buffer#set_string

closes #390