remove unnecessary depedency in test build
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?)
use the same error msg as mrb_str_to_cstr/CRuby

It's better to have one message for the same error
than two different (although similar worded) ones.
Fix extended arena check in gc_protect
use macros for assignments to ci->err
mrb_ary_splat() to call #to_a
quicker release of arena after ensure calls
always clear ci->err
pop ensure stack before execution; close #1638
should splat and iterate elements when "*ary" speficied in the case-when clause; close #1627
object_id may not return Fixnum (ISO says Integer), since intptr_t may be bigger than Fixnum, so type check for Numeric; ref #1630
Test case statement with splat
execute ensure clause correctly without OP_EPOP
avoid decrimenting eidx out of ecall()
need not to initialize ci->err at toplevel
Test for raise in ensure