mrdb: fix crash when stepping into Proc.call

update stable version number in README

How to reproduce:

```
cat a.rb
Proc.new { 1 }.call

echo step | mrdb a.rb
```

fix infinite loop in String#match(arg) when arg is String

Add String#upto

add String#ljust into mruby-string-ext

fix Segmentation fault on Struct#inspect due to recursive Struct object

Use sizeof() instead of strlen().

Properly implement directory tasks

Use mrb_exc_new_str_lit().

Removed duplicated declarations.

 * `mrb_show_version()`
 * `mrb_show_copyright()`

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]