Inconsistent signatures for `Environment.getAt/assingAt`

[pekkaklarck]

Signatures at Environment.java are:

Object getAt(int distance, String name)
void assignAt(int distance, Token name, Object value)

Using either String or Token as the type of name would fix the inconsistency. Using Token would also be consistency with get and assign which actually need a Token. Both getAt and assignAt would be fine also with String.