preserve significands in float-string conversion

Fix possible arena overflow in mruby-hast-ext.

add tests for String#capitalize!, String#downcase!, and String#upcase!

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

Added spaces in version.h macros for complying with C++11

iv_size (non-seglist): return 0 if t is NULL

fdigit may be negative due to error if mrb_float is 32bit (e.g. 10**36 on 32bit arch); ref #1713 #1714

fix mrb_flo_to_str() exponent problem

modify mrb_to_str() in order to display big float number

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.

keep stack with nlocals instead of nregs; fix #1706

cleanup warnings in hash.c

/home/fleuria/code/mruby/src/hash.c:159:1: warning: ‘mrb_hash_dp`defined but not used [-Wunused-function]
 mrb_hash_dup(mrb_state *mrb, mrb_value hash)
     ^

this commit defines Hash#dup to take advantage of mrb_hash_dup, however
it seems that Hash#dup is not in ISO standard.

hash / hash-ext: various small changes

MRUBY_VERSION should be only numbers separated by dot, like JRUBY_VERSION or Rubinius::VERSION