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

src/codegen/llvm.zig

@@ -2486,7 +2486,7 @@ pub const Object = struct {
                 var union_name_buf: ?[:0]const u8 = null;
                 defer if (union_name_buf) |buf| gpa.free(buf);
                 const union_name = if (layout.tag_size == 0) name else name: {
-                    union_name_buf = try std.fmt.allocPrintZ(gpa, "{s}:Payload", .{name});
+                    union_name_buf = try std.fmt.allocPrintSentinel(gpa, "{s}:Payload", .{name}, 0);
                     break :name union_name_buf.?;
                 };