tests for "case" expression.

adjust proc->target_class according to surrounding proc->target_class; based on a patch from @Fleurer; close #1627

Test for raise in ensure

Reduce unnecessary mrbgems full test rebuild.

Test case statement with splat

object_id may not return Fixnum (ISO says Integer), since intptr_t may be bigger than Fixnum, so type check for Numeric; ref #1630

use macros for assignments to ci->err

tiny word boxing optimization

remove superfluous return in codedump_all

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