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
fix mrb_flo_to_str() exponent problem
Merge pull request #1714 from h2so5/f2s

fix mrb_flo_to_str() exponent problem
fdigit may be negative due to error if mrb_float is 32bit (e.g. 10**36 on 32bit arch); ref #1713 #1714
use powf() instead of pow() on MRB_USE_FLOAT
small refactoring; direct return
remove max_digit from mrb_flo_to_str()
define FLO_EPSILON depends on float/double
remove trailing zeros from float string representation
use double instead of mrb_float (that may be single precision float) to reduce errors
iv_size (non-seglist): return 0 if t is NULL
Merge pull request #1717 from cremno/iv_size-t-null

iv_size (non-seglist): return 0 if t is NULL
Added spaces in version.h macros for complying with C++11
Merge pull request #1719 from pbosetti/master

Added spaces in version.h macros for complying with C++11
move src/error.h to include/mruby/error.h
Merge pull request #1720 from take-cheeze/move_error_h

move src/error.h to include/mruby/error.h