GenericReader error set fixes

add error.EndOfStream to readEnum() and isBytes() so that users can
catch these errors.  this also prevents them from panicing with
'invalid error value' on EndOfStream.

test both methods.

lib/std/io.zig

@@ -308,7 +308,7 @@ pub fn GenericReader(
             return @errorCast(self.any().skipBytes(num_bytes, options));
         }
 
-        pub inline fn isBytes(self: Self, slice: []const u8) Error!bool {
+        pub inline fn isBytes(self: Self, slice: []const u8) NoEofError!bool {
             return @errorCast(self.any().isBytes(slice));
         }
 
@@ -320,7 +320,7 @@ pub fn GenericReader(
             return @errorCast(self.any().readStructBig(T));
         }
 
-        pub const ReadEnumError = Error || error{
+        pub const ReadEnumError = NoEofError || error{
             /// An integer was read, but it did not match any of the tags in the supplied enum.
             InvalidValue,
         };

lib/std/io/test.zig

@@ -182,3 +182,16 @@ test "updateTimes" {
     try expect(stat_new.atime < stat_old.atime);
     try expect(stat_new.mtime < stat_old.mtime);
 }
+
+test "GenericReader methods can return error.EndOfStream" {
+    // https://github.com/ziglang/zig/issues/17733
+    var fbs = std.io.fixedBufferStream("");
+    try std.testing.expectError(
+        error.EndOfStream,
+        fbs.reader().readEnum(enum(u8) { a, b }, .Little),
+    );
+    try std.testing.expectError(
+        error.EndOfStream,
+        fbs.reader().isBytes("foo"),
+    );
+}