grapheme_chop now works on arrays like CORE::chop
grapheme_chop now works on hashes like CORE::chop
grapheme_length returns undef when passed undef

... like CORE::length
grapheme_reverse works more like CORE::reverse

+ works on lists of strings instead of just a single string
+ works just like reverse() in list context
no longer call utf8::upgrade in grapheme_*

if the user wants this functionality, she'll have to do it herself,
just like CORE:: functions.