Fixed float tolerance in tests when MRB_USE_FLOAT is set

Update AUTHORS

To prevent crash from nasty code like:

  class Time
    def initialize
    end
  end

  a = Time.new
  b = Time.new
  a.initialize_copy b

Fix Enumeable#take some incompatibility

Otherwise, C++ compilers will skip this constant when producing object files.

Fix segfault when Fixnum#chr doesn't return a string

Fix segfault in Array#sample

Fixed C declaration in re.h

mruby special.

Fixnum is usually big enough for shift width.