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/json/stringify.zig

@@ -689,7 +689,8 @@ fn outputUnicodeEscape(codepoint: u21, out_stream: anytype) !void {
         // then it may be represented as a six-character sequence: a reverse solidus, followed
         // by the lowercase letter u, followed by four hexadecimal digits that encode the character's code point.
         try out_stream.writeAll("\\u");
-        try std.fmt.formatIntValue(codepoint, "x", std.fmt.FormatOptions{ .width = 4, .fill = '0' }, out_stream);
+        //try w.printInt("x", .{ .width = 4, .fill = '0' }, codepoint);
+        try std.fmt.deprecatedFormat(out_stream, "{x:0>4}", .{codepoint});
     } else {
         assert(codepoint <= 0x10FFFF);
         // To escape an extended character that is not in the Basic Multilingual Plane,
@@ -697,9 +698,11 @@ fn outputUnicodeEscape(codepoint: u21, out_stream: anytype) !void {
         const high = @as(u16, @intCast((codepoint - 0x10000) >> 10)) + 0xD800;
         const low = @as(u16, @intCast(codepoint & 0x3FF)) + 0xDC00;
         try out_stream.writeAll("\\u");
-        try std.fmt.formatIntValue(high, "x", std.fmt.FormatOptions{ .width = 4, .fill = '0' }, out_stream);
+        //try w.printInt("x", .{ .width = 4, .fill = '0' }, high);
+        try std.fmt.deprecatedFormat(out_stream, "{x:0>4}", .{high});
         try out_stream.writeAll("\\u");
-        try std.fmt.formatIntValue(low, "x", std.fmt.FormatOptions{ .width = 4, .fill = '0' }, out_stream);
+        //try w.printInt("x", .{ .width = 4, .fill = '0' }, low);
+        try std.fmt.deprecatedFormat(out_stream, "{x:0>4}", .{low});
     }
 }