Add a couple Fiber tests

add Fiber#alive?

Add MRB_WORD_BOXING mode (represent mrb_value as a word)

Add Fiber Test for issue #1264

should have protected the return value from GC, even when the method is defined in Ruby; close #1263

Change Fibder to Fiber

Change fortmat to format.

mrbc: use binary mode on outfile

This solves the problem of incorrect bytecode generated by mrbc
for single files on Windows platform (which is by default text-mode)

removed unused variables / assigns never used