remove superfluous includes

- reduce compile time by a little bit (full-core: ~0.7s for me)
- thanks to 'include-what-you-use' for some help
- include Standard C header files before any other (coding style)

Remove duplicate test for mruby-string-utf8

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