std.Io: delete GenericReader, AnyReader, FixedBufferStream; and related API breakage

[andrewrk]

Follow-up from #24329.

This patchset is the last in the Writergate series. With this, the purification of std.Io is complete, and it is now an empty vessel, ready to accept the new Io interface.

std.Io.Writer.Allocating & Alignment

This API now has a new field:

alignment: std.mem.Alignment,

This is a runtime-known alignment value. The Allocator API supports this if you use the "raw" function variants.

I tried this out in order to be able to reuse std.Io.Writer.Allocating in a place that needed alignment. It worked out pretty well. So well in fact, that this might be an opportunity to do something similar to std.array_list.Aligned...

Migration Guide

• std.io -> std.Io
• std.Io.GenericReader -> std.Io.Reader
• std.Io.AnyReader -> std.Io.Reader
• std.leb.readUleb128 -> std.Io.Reader.takeLeb128
• std.leb.readIleb128 -> std.Io.Reader.takeLeb128

FixedBufferStream (reading)

    var fbs = std.io.fixedBufferStream(data);
    const reader = fbs.reader();

⬇️

    var reader: std.Io.Reader = .fixed(data);

FixedBufferStream (writing)

    var fbs = std.io.fixedBufferStream(buffer);
    const writer = fbs.writer();

⬇️

    var writer: std.Io.Reader = .fixed(buffer);

std.fs.Dir.readFileAlloc

-    const contents = try std.fs.cwd().readFileAlloc(allocator, file_name, 1234);
+    const contents = try std.fs.cwd().readFileAlloc(file_name, allocator, .limited(1234));

Note that the limit has a difference; if it's reached it also returns the error. Also the error has been changed from FileTooBig to StreamTooLong.

std.fs.File.readToEndAlloc

const contents = try file.readToEndAlloc(allocator, 1234);

⬇️

var file_reader = file.reader(&.{});
const contents = try file_reader.interface.allocRemaining(allocator, .limited(1234));