New and Improved io.Stream interface

[gingerBill]

This is a big change which changes the io.Stream interface from being a hybrid inheritance model (pointer-to-vtable + data) to a flat model (procedure-value + data) which is more akin to a runtime.Allocator.

The number of stream "modes" has been greatly reduced to remove unnecessary and confusing options:

Stream_Mode :: enum {
	Close,
	Flush,
	Read,
	Read_At,
	Write,
	Write_At,
	Seek,
	Size,
	Destroy,
	Query, // query what modes are available
}

Stream_Proc :: #type proc(stream_data: rawptr, mode: Stream_Mode, p: []byte, offset: i64, whence: Seek_From) -> (n: i64, err: Error)

Stream :: struct {
	procedure: Stream_Proc,
	data:      rawptr,
}

The io.Writer etc types are now just aliases of io.Stream rather than being distinct data types.

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In core:os/os2, the File type now uses the io.Stream internally for its general operations meaning a stream can pretend to be a file if necessary (similar to open_memstream).