Move generic functions to grn_plugin_* from grn_tokenizer_*

grn_tokenizer_charlen() ->
grn_plugin_charlen()

grn_tokenizer_isspace() ->
grn_plugin_isspace()

They are not tokenizer specific functions. They are useful for
normalizer. So the prefix is changed to grn_plugin_.

grn_tokenizer_*() still exists for backward compatibility. Use
grn_plugin_*() for newly written code.