Moved file and path related methods from StringHelper to FileHelper, renamed StringHelper byte methods not to be misused as string methods

- allows to have dynamic definition of attributes depended on the
  instance
- there was no real need for it to be static. Places that used it static
  have been refactored.

fixes #1318

+ added tests and fixed behavior

this allows us to implement other activerecord implementations based on
NoSQL dbms

[ci skip]

fixes #842

- allows checking whether a relation has been populated
- getting a list of relation names that have been populated
- getting all populated relation data

todo:

- [] add phpdoc

... for different dbms

this is to make relations work. tests are passing now.
refactoring needed to remove the dependency

- introduced RecordSchema class for schema definition