Антон Белый, 2538

Implemented:
 - Time- and memory-efficient expression parser with linear complexity.
 - Tests for checking parsing correctness, calculation correctness, and rudimentary performance regression tests with randomly generated expressions.
 - Nice and convenient UI, supporting:
  * Hardware keyboard
  * Brackets autoclosing
  * Calculating expressions on the go
  * Separate layout for horizontal device orientation

Room for improvement:
 - Current expression is erased from screen after rotation
 - Parser is not truly online: it could parse only changed part of expression, instead it re-parses the whole expression again