Represent source data as blocks or expressions

[nergal-perm]

In order to make the core independent of source table data I have to introduce a layer of abstractions, that will make possible to plug in different readers, like from yaml files, for example.

My initial idea is to transform the source data into some Expressions, that could have a type, a left-hand side and a right-hand side. The operator could be also extracted into a separate field.