rework mruby-objectspace and gc.[ch]

- functions should have C linkage (for C++ code)
- add prefix to each_object_callback
- use more appropriate variable types / initialization
- ObjectSpace::count_objects has 1 opt. arg.
- prefer mrb_intern_lit to mrb_intern_cstr for str. lit.
- mruby/value.h is included by mruby.h
- adjust coding style

clean up external symbols

update copyright year

etc.c: small cleanup

remove mrb_str_format declaration

remove unused and unneeded:
- sysexit_status
- type (a global variable)

add mrb_ prefix to:
- codedump_all
- class_instance_method_list
- parser_dump

make various functions static, incl.:
- yyparse
- make_exception

The function mrb_str_format is only defined when the mrbgem mruby-sprintf was
activated. That might not always be the case (by default it currently is).
mrb_str_format still has external linkage, so from now on mrbgem authors
have to declare it in their source code if they want or need to call it.

Let's not wait until May this time
(f0a9b95a)!

mrb_data_object_alloc: unnecessary cast
mrb_lastline_get: mruby doesn't support $_, weird and unused code
mrb_exec_recursive: see 4e46abb8
mrb_block_proc: unused, doesn't really do anything (or needs to be
rewritten)
mrb_obj_to_sym: actually use id (and MSVC detected unreachable code)

more accurate backtrace lineno

clone Class/Module rightly

fix utf8 codepage

escape non-ascii characters correctly in String#inspect

Increase test coverage of mruby-struct

Added rewrite of Array#[] to mruby-array-ext gem

String#length should be an alias of String#size

with Ranges (as a[1..-2])

add operator "!~".

fix ISO reference number of String#=~.