`cpp` does not raise error on undefined macro access in condition.

You had to define this macro on big endian platforms, but it is very
error-prone. So define the macro automatically if possible.

The `MRB_ENDIAN_BIG` macro is originally used for `NaN` boxing.
We cannot assume it is defined on every big endian platform (#4190
is the case). So instead of relying on untrusted `MRB_ENDIAN_BIG`, we
use `BYTE_ORDER` macro with a fallback function to check endian in
runtime.

Fix parenthesis for macros

Android bionic defines `TIME_UTC` but does not provide `timespec_get`.

Make mrb_ary_clear() function callable from C again

We have removed implicit conversion to strings using `to_int`. But some
users still using `to_int` as a typical integer method, i.e. they do
string check by code like: `obj.respond_to?(:to_int)`. So we have
recovered the method.

We have removed implicit conversion to strings using `to_str`. But some
people still using `to_str` as a typical string method, i.e. they do
string check by code like: `obj.respond_to?(:to_str)`. So we have
recovered the method.

Add `mrb_hash_size()` function

Fix sleep example

Release mruby 2.0.0

* return `void` instead of `mrb_bool'.
* non zero return value from `func` breaks the loop.
* no longer remove items on negative return value from `func`.

Adds Module#< and Module#<=