Fix extended arena check in gc_protect

use the same error msg as mrb_str_to_cstr/CRuby

remove various preprocessor conditionals

NaN boxing: use uintptr_t for bit manipulations

fix some warnings emitted by 'ruby -w'

It's better to have one message for the same error
than two different (although similar worded) ones.

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

- File.exists? is deprecated since 2.1.0
- tasks/mruby_build_commands.rake:268: ambiguous argument

More kernel testing

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

add String#partition and String#rpartition.

More tests for Class behavior

add tests for Array methods

The return-value of mrb_run in top-level-scope
should be the evaluated value at last.

Merge branch 'singletonfix' of https://github.com/carsonmcdonald/mruby into carsonmcdonald-singletonfix

Change :vattr= to :cattr= in respond_to? test