Give the type 'void' to functions have no argument.

According to the C standard,
it is desirable to give the type 'void'
to functions have no argument.
Merge pull request #1709 from cubicdaiya/issues/nonvoid

Give the type 'void' to functions have no argument.
modify mrb_to_str() in order to display big float number
normalize NaN after division that may generate NaN; fix #1712
Merge pull request #1710 from takkaw/flo_to_str

modify mrb_to_str() in order to display big float number
Merge branch 'master' into iij