Improvement request: Closure

Is is better to use php Closure and not to write php code in strings? This issue is the only one that stops me to use soft-mocks.

**As it is now:**
```
\Badoo\SoftMocks::redefineFunction(
'strlen',
'$a',
'return \\Badoo\\SoftMocks::callOriginal("strlen", [trim($a)]));'
);
```

**Can be with closure:**

```
\Badoo\SoftMocks::redefineFunction( 'strlen', function( $a ) {
return \Badoo\SoftMocks::callOriginal('strlen', [trim($a)]);
}) ;
```

Start at the SoftMocks::redefineFunction entry point and trace how its current string-based arguments are handled. The work is done when the closure form shown in the issue can redefine strlen and call the original function with the trimmed argument, while the existing string form continues to work.

Written by the indexing model from the issue text.