array implementation: several small changes

Rakefile: make bin directory

Clang and ICC/ICL: define DIRECT_THREADED

mruby-math: remove Cygwin compatibility macros

src/array.c:
 - make various functions without declaration in the mruby headers static
 - removed all uncessary int casts and two useless comments
 mrb_ary_new_from_values: move to similar functions
 mrb_check_array_type: fix indentation

include/mruby/array.h:
 mrb_shared_array: padding (default "mrbconf.h" on an usual 64-bit system)
                   sizeof(mrb_int)==sizeof(int)==4 && sizeof(mrb_value*)==8

both:
 mrb_ary_aget:  remove declaration in header and make static
                nobody uses it which is not surprising since it has 1 req arg!
 mrb_assoc_new: small optimization and move to similar functions
 mrb_ary_len:   re-implement as static inline function (it is used by mrbgems)
                programmers should directly use RARRAY_LEN instead

nan:
 The first non-test release of Cygwin 1.5.x was over a decade ago:
 <http://cygwin.com/ml/cygwin-announce/2003-09/msg00001.html>

log/log10:
 This workaround is not needed anymore (1.2.6.1, 1.3 - both over 6 years ago):
 <https://sourceware.org/cgi-bin/cvsweb.cgi/src/newlib/libm/math/w_log.c?cvsroot=src>

This is most likely only needed on Windows be-
cause __GNUC__ is not defined by both compilers.

It might fail with some unusual configurations.
If that is the case, feel free to open an issue.

Fix Kernel#global_variables for $1-$9

use mrb_bool, FALSE and TRUE more

Remove *very early version* notice

Add download link for stable version to README

- add download link of stable version 1.0.0
- remove remark for mruby branch due to non-existence

Remove unnecessary semicolon

Pass forgotten variable gem_flags_after_libraries to tool's linker.run.