Implement string format method

## python
```
>>> a = 'alpha'
>>> b = 'bravo'
>>> d = 'delta'
>>> origin = '{} {} charlie {}'
>>> new = origin.format(a, b, d)
>>> print(new)
alpha bravo charlie delta
>>> print(origin)
{} {} charlie {}
>>>
```

## gpython
```
>>> a = 'alpha'
>>> b = 'bravo'
>>> d = 'delta'
>>> origin = '{} {} charlie {}'
>>> new = origin.format(a, b, d)
Traceback (most recent call last):
File "", line 1, in
FIXME line of source goes here
AttributeError: "'str' has no attribute 'format'"
>>> print(new)
Traceback (most recent call last):
File "", line 1, in
FIXME line of source goes here
NameError: "name 'new' is not defined"
>>> print(origin)
{} {} charlie {}
>>>
```

Attribute error occured using string.