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

adding mruby-string-utf8 breaks String#[float]

fix ISBLANK() for Visual Studio < 2013

Visual Studio versions older than 2013 lack C99's isblank().
Since only ASCII characters are passed to it, implement it
directly without calling the locale-specific isblank().

adhoc fix for pass build

Fix: Numeric#step infinite loop.

mrdb: add missing null-termination

mrdb: fix two memory leaks

mrdb: minor changes