Cleanup around mrb_raise calls

Add fib39.rb to benchmark/

Fix the calc_crc_16_ccitt signature to match the source file

Remove the bit-field by mrb_value.tt

Cleanup numeric.c

Add null char terminate to each symbol name.

 Export mrb_flo_to_str() as API.

 Add new API mrb_flo_to_fixnum(). You can replace mrb_flt2big() to mrb_flo_to_fixnum() with few modifications.

Fixed testrb load path

Remove API mrb_string_value(). 

Remove API mrb_string_value(). There have mrb_str_to_str() in the core. And mrb_string_value() is no merit to keep using.

Display the command line when mrbc is kicked in the verbose mode.

Simplify dump/load

 It is not used in the core.
 It does not have mrb_ prefix.
 strtol() is slightly heavy and we have similar API.

Modify the type of line-number to uint16_t. Type short is not portable. And it cannot be more than UINT16_MAX because of the mrbc binary format.