Add arithmetical expressions support.

Precedence order for operators:

  * logical: `not`, `and`, `or` and `xor`,
  * user-defined,
  * arithmetical: `+`, `-` (or `−`), `*` (or `×`), `/` (or `÷`), `**`
    and `%`.

Also support scientist notation (for example 4.2e-7).
Add all operators, fix typos and re-layout.