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

build.zig

@@ -279,7 +279,7 @@ pub fn build(b: *std.Build) !void {
 
                 const ancestor_ver = try std.SemanticVersion.parse(tagged_ancestor);
                 if (zig_version.order(ancestor_ver) != .gt) {
-                    std.debug.print("Zig version '{}' must be greater than tagged ancestor '{}'\n", .{ zig_version, ancestor_ver });
+                    std.debug.print("Zig version '{f}' must be greater than tagged ancestor '{f}'\n", .{ zig_version, ancestor_ver });
                     std.process.exit(1);
                 }
 
@@ -1449,7 +1449,7 @@ fn generateLangRef(b: *std.Build) std.Build.LazyPath {
     }
 
     var dir = b.build_root.handle.openDir("doc/langref", .{ .iterate = true }) catch |err| {
-        std.debug.panic("unable to open '{}doc/langref' directory: {s}", .{
+        std.debug.panic("unable to open '{f}doc/langref' directory: {s}", .{
             b.build_root, @errorName(err),
         });
     };
@@ -1470,7 +1470,7 @@ fn generateLangRef(b: *std.Build) std.Build.LazyPath {
             // in a temporary directory
             "--cache-root", b.cache_root.path orelse ".",
         });
-        cmd.addArgs(&.{ "--zig-lib-dir", b.fmt("{}", .{b.graph.zig_lib_directory}) });
+        cmd.addArgs(&.{ "--zig-lib-dir", b.fmt("{f}", .{b.graph.zig_lib_directory}) });
         cmd.addArgs(&.{"-i"});
         cmd.addFileArg(b.path(b.fmt("doc/langref/{s}", .{entry.name})));