Changes

Introduce special ForeignVariable to model a unified access to variables from any context. 
For example it will allow to transparently use temps from the local context in doIt expressions:
```Smalltalk
| temp |
temp := 100.
foreignVariable := ForeignVariable named: #temp fromContext: thisContext.
 Smalltalk compiler
	bindings: {foreignVariable};
	evaluate: '1+temp'.
```