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/debug/Dwarf.zig

@@ -2302,11 +2302,7 @@ pub const ElfModule = struct {
                 };
                 defer debuginfod_dir.close();
 
-                const filename = std.fmt.allocPrint(
-                    gpa,
-                    "{s}/debuginfo",
-                    .{std.fmt.fmtSliceHexLower(id)},
-                ) catch break :blk;
+                const filename = std.fmt.allocPrint(gpa, "{x}/debuginfo", .{id}) catch break :blk;
                 defer gpa.free(filename);
 
                 const path: Path = .{
@@ -2330,12 +2326,8 @@ pub const ElfModule = struct {
                 var id_prefix_buf: [2]u8 = undefined;
                 var filename_buf: [38 + extension.len]u8 = undefined;
 
-                _ = std.fmt.bufPrint(&id_prefix_buf, "{s}", .{std.fmt.fmtSliceHexLower(id[0..1])}) catch unreachable;
-                const filename = std.fmt.bufPrint(
-                    &filename_buf,
-                    "{s}" ++ extension,
-                    .{std.fmt.fmtSliceHexLower(id[1..])},
-                ) catch break :blk;
+                _ = std.fmt.bufPrint(&id_prefix_buf, "{x}", .{id[0..1]}) catch unreachable;
+                const filename = std.fmt.bufPrint(&filename_buf, "{x}" ++ extension, .{id[1..]}) catch break :blk;
 
                 for (global_debug_directories) |global_directory| {
                     const path: Path = .{

lib/std/debug/Pdb.zig

@@ -395,7 +395,7 @@ const Msf = struct {
     streams: []MsfStream,
 
     fn init(allocator: Allocator, file: File) !Msf {
-        const in = file.reader();
+        const in = file.deprecatedReader();
 
         const superblock = try in.readStruct(pdb.SuperBlock);
 
@@ -514,7 +514,7 @@ const MsfStream = struct {
         var offset = self.pos % self.block_size;
 
         try self.in_file.seekTo(block * self.block_size + offset);
-        const in = self.in_file.reader();
+        const in = self.in_file.deprecatedReader();
 
         var size: usize = 0;
         var rem_buffer = buffer;