modify mrb_to_str() in order to display big float number
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