Patch: add conversion from pg_wchar to multibyte

Hackers,

attached patch adds conversion from pg_wchar string to multibyte string.
This functionality is needed for my patch on index support for regular
expression search
http://archives.postgresql.org/pgsql-hackers/2011-11/msg01297.php .
Analyzing conversion from multibyte to pg_wchar I found following types of
conversion:
1) Trivial conversion for single-byte encoding. It just adds leading zeros
to each byte.
2) Conversion from UTF-8 to unicode.
3) Conversions from euc* encodings. They write bytes of a character to
pg_wchar in inverse order starting from lower byte (this explanation assume
little endian system).
4) Conversion from mule encoding. This conversion is unclear for me and
also seems to be lossy.

It was easy to write inverse conversion for 1-3. I've changed 4 conversion
to behave like 3. I'm not sure my change is ok, because I didn't understand
original conversion.

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With best regards,
Alexander Korotkov.