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/compiler/objcopy.zig

@@ -635,11 +635,11 @@ const HexWriter = struct {
             const payload_bytes = self.getPayloadBytes();
             assert(payload_bytes.len <= MAX_PAYLOAD_LEN);
 
-            const line = try std.fmt.bufPrint(&outbuf, ":{0X:0>2}{1X:0>4}{2X:0>2}{3s}{4X:0>2}" ++ linesep, .{
+            const line = try std.fmt.bufPrint(&outbuf, ":{0X:0>2}{1X:0>4}{2X:0>2}{3X}{4X:0>2}" ++ linesep, .{
                 @as(u8, @intCast(payload_bytes.len)),
                 self.address,
                 @intFromEnum(self.payload),
-                std.fmt.fmtSliceHexUpper(payload_bytes),
+                payload_bytes,
                 self.checksum(),
             });
             try file.writeAll(line);
@@ -1495,7 +1495,7 @@ const ElfFileHelper = struct {
         if (size < prefix.len) return null;
 
         try in_file.seekTo(offset);
-        var section_reader = std.io.limitedReader(in_file.reader(), size);
+        var section_reader = std.io.limitedReader(in_file.deprecatedReader(), size);
 
         // allocate as large as decompressed data. if the compression doesn't fit, keep the data uncompressed.
         const compressed_data = try allocator.alignedAlloc(u8, .@"8", @intCast(size));