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#<=

e.g.
```
def m(a,(b,c),d); p [a,b,c,d]; end
m(1,[2,3],4)  # => [1,2,3,4]
```

mruby limitation:
Destructured arguments (`b` and `c` in above example) cannot be accessed
from the default expression of optional arguments and keyword arguments,
since actual assignment is done after the evaluation of those default
expressions. Thus:

```
def f(a,(b,c),d=b)
  p [a,b,c,d]
end
f(1,[2,3])
```

raises `NoMethodError` for `b` in mruby.

It can happen if signals are used (e.g. from `mruby-alarm` gem).

Because it can swallow `break` etc. if they are used in loops.

Add documentation to mrb_load_irep