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/std/Build/Step/InstallDir.zig

@@ -65,7 +65,7 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
     const src_dir_path = install_dir.options.source_dir.getPath3(b, step);
     const need_derived_inputs = try step.addDirectoryWatchInput(install_dir.options.source_dir);
     var src_dir = src_dir_path.root_dir.handle.openDir(src_dir_path.subPathOrDot(), .{ .iterate = true }) catch |err| {
-        return step.fail("unable to open source directory '{}': {s}", .{
+        return step.fail("unable to open source directory '{f}': {s}", .{
             src_dir_path, @errorName(err),
         });
     };