[Parser] Reuse expressions in syntax tree

[sys27]

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.