add function 'mrb_str_new_cstrl'

This is length-limited 'mrb_str_new_cstr' and comparable to 'lua_pushlstirng' of Lua.(In this case, 'mrb_str_new_cstr' is comparable to 'lua_pushstring' of Lua)

## use case

As I'm in trouble using 'mrb_str_new_cstr' to the length-attached string of nginx(ngx_str_t), I want to add this function to mruby for developing ngx_mruby.

For, ngx_str_t is defined as following.

```c
typedef struct {
    size_t      len;
    u_char     *data;
} ngx_str_t;
```

As the data is not ensured zero-terminated in Nginx, the following code does not works as I had expected.

```c
/* This is the value when URI is "/mruby".
/* r->method_name.data -> GET /mruby/ HTTP/1.1  */
/* r->method_name.len -> 3 */
mrb_value val;
ngx_http_request_t *r;
r = ngx_mrb_get_request();
val = mrb_str_new_cstr(mrb, (const char *)r->method_name.data); /* "GET /mruby/ HTTP/1.1" is copied */
```

As r->method_name.data is not null-terminated, 
the actual method name becomes not "GET" but "GET /mruby/ HTTP/1.1".

mrb_str_new_cstrl resolves this problem.

```c
/* This is the value when URI is "/mruby".
/* r->method_name.data -> GET /mruby/ HTTP/1.1  */
/* r->method_name.len -> 3 */
mrb_value val;
ngx_http_request_t *r;
r = ngx_mrb_get_request();
val = mrb_str_new_cstrl(mrb, (const char *)r->method_name.data, r->method_name.len); /* "GET" is copied */
```