[rfc] unicode escapes for extended strings

Seems I'm bad at communicating in english, so here is C variant of
my proposal to bring \u escaping into extended strings.  Reasons:

- More people are familiar with \u escaping, as it's standard
  in Java/C#/Python, probably more..
- U& strings will not work when stdstr=off.

Syntax:

  \uXXXX      - 16-bit value
  \UXXXXXXXX  - 32-bit value

Additionally, both \u and \U can be used to specify UTF-16 surrogate
pairs to encode characters with value > 0xFFFF.  This is exact behaviour
used by Java/C#/Python.  (except that Java does not have \U)


I'm ok with this patch left to 8.5.

--
marko