fixed error-handling for mrb_open().

When mrb_open() is called again, it is not checked.

Round execution time

Set correct build dir for bins

Remove redundant NULL checks for mrb_malloc().

Avoid block_given? in mrbgem:enum.rb to reduce method calls

Use suitable type.

mruby-eval: fix typo

Fix splat and multiple assignments

Case1: From variable

Code:

    a = [1, 2, 3, 4, 5]
    b, c, *d = a

    p [a, b, c, d]

Before:

    [[1, 2, 3, 4, 5], 1, 2, []]

After:

    [[1, 2, 3, 4, 5], 1, 2, [3, 4, 5]]

Ruby:

    [[1, 2, 3, 4, 5], 1, 2, [3, 4, 5]]

Case2: From variables

Code:

    a = [1, 2, 3]
    b = [4, 5, 6, 7]
    c, d, *e, f, g = *a, *b

    p [a, b, c, d, e, f, g]

Before:

    [[1, 2, 3], [4, 5, 6, 7], 1, 2, [], 6, 7]

After:

    [[1, 2, 3], [4, 5, 6, 7], 1, 2, [3, 4, 5], 6, 7]

Ruby:

    [[1, 2, 3], [4, 5, 6, 7], 1, 2, [3, 4, 5], 6, 7]

Case 3: "for"

Code:

    a = [1, 2, 3, 4, 5, 6, 7]
    for b, c, *d, e, f in [a] do
      p [a, b, c, d, e, f]
    end

Before:

    [[1, 2, 3, 4, 5, 6, 7], 1, 2, [], nil, nil]

After:

    [[1, 2, 3, 4, 5, 6, 7], 1, 2, [3, 4, 5], 6, 7]

Ruby:

    [[1, 2, 3, 4, 5, 6, 7], 1, 2, [3, 4, 5], 6, 7]

mrdb: fix that break command cannot handle Windows paths

Before fix:
```
$ mrdb c:\tmp\b.rb
(c:\tmp\b.rb:1) break c:\tmp\b.rb:3
Class name 'c' is invalid.
```

After fix:
```
$ mrdb c:\tmp\b.rb
(c:\tmp\b.rb:1) break c:\tmp\b.rb:3
Breakpoint 1: file c:\tmp\b.rb, line 3.
```

avoid a loop to find parent's callinfo using mrb->c->cibase[env->cioff]

allocation size of irep->iv is irep->nlocals-1.

Add String#prepend

Add String#insert

Fix crash if #inspect does not return a string value

case 1
~~~
class A; def inspect; 1; end; end
A.new.a
~~~

case 2

~~~
class A
  def self.inspect
    1
  end
  alias_method :a, :b
end
~~~

fix usage message