Separate unicode formatting into its own fmt_unicode function.
Remove the fmtUnicode wrapper and the f.unicode and f.uniQuote
flags that are not needed anymore. Remove mangling and restoring
of the precision and sharp flags.

Removes the buffer copy needed for %#U by moving
the character encoding before the number encoding.

Changes the behavior of plus and space flag to have
no effect instead of printing a plus or space before "U+".

Always print at least four digits after "U+"
even if precision is set to less than 4.

Change-Id: If9a0ee79e9eca2c76f06a4e0fdd75d98393899ac
Reviewed-on: https://go-review.googlesource.com/20574
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>