have MARC::Charset::Code->marc_value() handle G0/G1 conversion

Since there's at present no need to do things like have
ANSEL be the G0 character set when converting from UTF8 to
MARC8, this commit centralizes the logic for deciding
whether to return the G0 or G1 MARC8 representation of a
character.

Also add MARC::Charset::Code->g0_marc_value(), which returns
the G0 representation of the character for use by the
character DB.

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>
final changelog update for MARC::Charset 1.35

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>