Remove `Struct` defined check in test.
Add document to Fiber#transfer .
Remove unnecessary `require 'fiber'` note in Fiber.current document.
Merge pull request #2102 from take-cheeze/fiber_doc

Improve mruby-fiber document.
Merge pull request #2101 from take-cheeze/struct_test

Remove `Struct` defined check in test.
Also set $0 for binary files
remove `mrb_str_buf_cat`

It does the same as `mrb_str_cat`.
Merge pull request #2103 from titouanc/global-zero-bytecode

Also set $0 for binary files
Merge pull request #2104 from cremno/remove-mrb_str_buf_cat

remove `mrb_str_buf_cat`