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
Merge pull request #1673 from cremno/rework-objspace-gc

rework mruby-objectspace and gc.[ch]
fix NODE_SPLAT codegen
Merge pull request #1675 from h2so5/node-splat-codegen

fix NODE_SPLAT codegen
add Float#nan?
Merge pull request #1678 from iij/float-nan-p

add Float#nan?
fibers cannot cross C function boundary; close #1680
Remove warning
Merge pull request #1682 from mattn/remove_warning

Remove warning
better codedump format
Fiber.new{break} caused SEGV
disable LocalJumpError test since the code causes SyntaxError (not LocalJumpError) on CRuby
add RUBY_VERSION to notify syntax compatible version
pass gem_flags_after_libraries
Merge pull request #1683 from take-cheeze/gem_flags_after_libraries

Pass forgotten variable gem_flags_after_libraries to tool's linker.run.
Add pluggable versioning by mruby-version
Fix release date
replace Kernel.show_version with MRUBY_DESCRIPTION
Add global constant MRUBY_COPYRIGHT
Fix MRUBY_BIRTH_YEAR