return-value of mrb_run is invalid in top-level-scope

The return-value of mrb_run in top-level-scope
should be the evaluated value at last.
add String#partition and String#rpartition.
More singleton tests
When superclass isn't a class tests
add some tests for [], delete_at, index, and rindex
Merge pull request #1629 from akuroda/add_array_test

add tests for Array methods
Merge pull request #1626 from carsonmcdonald/moreclasstests

More tests for Class behavior
Merge pull request #1625 from iij/pr-string-partition

add String#partition and String#rpartition.
Merge pull request #1624 from cubicdaiya/issues/invalid_return_in_top_level

return-value of mrb_run is invalid in top-level-scope
More object_id test coverage
Test coverage of extending stack and overflow
Method missing and inspect tests
Merge pull request #1630 from carsonmcdonald/morekerneltests

More kernel testing
successful recursion check to be shallower; ref #1630
return value from #object_id may not be Fixnum; ref #1630
fix some warnings emitted by 'ruby -w'

- File.exists? is deprecated since 2.1.0
- tasks/mruby_build_commands.rake:268: ambiguous argument
codegen.c: remove superfluous return
NaN boxing: use uintptr_t for bit manipulations
tiny word boxing optimization
remove various preprocessor conditionals

- HAVE_IEEEFP_H is nowhere defined or needed at all
- FreeBSD < 4 is unsupported since years
- MSVC workaround (around what exactly?)