std.fmt: breaking API changes

added adapter to AnyWriter and GenericWriter to help bridge the gap
between old and new API

make std.testing.expectFmt work at compile-time

std.fmt no longer has a dependency on std.unicode. Formatted printing
was never properly unicode-aware. Now it no longer pretends to be.

Breakage/deprecations:
* std.fs.File.reader -> std.fs.File.deprecatedReader
* std.fs.File.writer -> std.fs.File.deprecatedWriter
* std.io.GenericReader -> std.io.Reader
* std.io.GenericWriter -> std.io.Writer
* std.io.AnyReader -> std.io.Reader
* std.io.AnyWriter -> std.io.Writer
* std.fmt.format -> std.fmt.deprecatedFormat
* std.fmt.fmtSliceEscapeLower -> std.ascii.hexEscape
* std.fmt.fmtSliceEscapeUpper -> std.ascii.hexEscape
* std.fmt.fmtSliceHexLower -> {x}
* std.fmt.fmtSliceHexUpper -> {X}
* std.fmt.fmtIntSizeDec -> {B}
* std.fmt.fmtIntSizeBin -> {Bi}
* std.fmt.fmtDuration -> {D}
* std.fmt.fmtDurationSigned -> {D}
* {} -> {f} when there is a format method
* format method signature
  - anytype -> *std.io.Writer
  - inferred error set -> error{WriteFailed}
  - options -> (deleted)
* std.fmt.Formatted
  - now takes context type explicitly
  - no fmt string

lib/docs/wasm/markdown.zig

@@ -160,7 +160,7 @@ fn mainImpl() !void {
     var doc = try parser.endInput();
     defer doc.deinit(gpa);
 
-    var stdout_buf = std.io.bufferedWriter(std.fs.File.stdout().writer());
+    var stdout_buf = std.io.bufferedWriter(std.fs.File.stdout().deprecatedWriter());
     try doc.render(stdout_buf.writer());
     try stdout_buf.flush();
 }

lib/docs/wasm/markdown/renderer.zig

@@ -1,6 +1,7 @@
 const std = @import("std");
 const Document = @import("Document.zig");
 const Node = Document.Node;
+const assert = std.debug.assert;
 
 /// A Markdown document renderer.
 ///
@@ -229,18 +230,11 @@ pub fn renderInlineNodeText(
     }
 }
 
-pub fn fmtHtml(bytes: []const u8) std.fmt.Formatter(formatHtml) {
+pub fn fmtHtml(bytes: []const u8) std.fmt.Formatter([]const u8, formatHtml) {
     return .{ .data = bytes };
 }
 
-fn formatHtml(
-    bytes: []const u8,
-    comptime fmt: []const u8,
-    options: std.fmt.FormatOptions,
-    writer: anytype,
-) !void {
-    _ = fmt;
-    _ = options;
+fn formatHtml(bytes: []const u8, writer: *std.io.Writer) std.io.Writer.Error!void {
     for (bytes) |b| {
         switch (b) {
             '<' => try writer.writeAll("&lt;"),