[Parser] Reuse expressions in syntax tree

`2 + 2` is parsed as 
```
   +
 /   \
2     2
```
but it can be parsed as
```
   +
 /   \
 \   /
   2
```

Because expressions are immutable (#331) it can be applied to all types of expressions, but probably we can use simpler approach, apply it only for numbers and variables.

Related #146.