fix LockingTextReader: issues 13686 and 12320

[Issue 13686 (Reading unicode string with readf ("%s") produces a wrong
string)](https://issues.dlang.org/show_bug.cgi?id=13686) is fixed by reading all chars of a multibyte sequence and decoding.
Before, each char was mistaken for a dchar.

[Issue 12320 (std.stdio.LockingTextReader populates .front in .empty)](https://issues.dlang.org/show_bug.cgi?id=12320) is
fixed by moving the work from empty to popFront.
Yes.

> UTF-8 encodes each of the 1,112,064 valid code points in the Unicode code space (1,114,112 code points minus 2,048 surrogate code points) using one to four 8-bit bytes (http://en.wikipedia.org/wiki/UTF8)

std.utf.encode also takes a `char[4]`.