zlib (rb_gzreader_getc): localize and return cbuf directly

No point in having a long-lived cbuf in "struct gzfile"
since GZFILE_CBUF_CAPA is smaller than RSTRING_EMBED_LEN_MAX
(even on 32-bit).  We can also have rb_econv_convert write
directly to the return value instead of an intermediate buffer.

This brings "struct gzfile" from 264 to 256 bytes on 64-bit
systems to avoid taking an additional cache line.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63993 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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