update: keep result of scalar operation in tree itself

Now result is stored in operation itself. It is inconvenient if we are
going to apply multiple operations on the same field. To keep the said
result let's introduce scalar type that can hold all possible scalar
including strings.

By the way it is good time to get rid of `new_field_len` field. It is
excessive now.

Part of #8658

NO_TEST=refactoring
NO_DOC=refactoring
NO_CHANGELOG=refactoring