* Split `tok::number` token into `tok::real` and `tok::integer`
* Add new tokens: `tok::tilde`, `tok::arrow` and `tok::kinetic`
  (keyword)
* Add new expressions: `IntegerExpression` (deriving from
  `NumberExpression`), `ReactionExpression`, `StoichExpression`
  and `StoichTermExpression`.
* Add new `procedureKind`: `kinetic`.
* Add parse rules for KINETIC blocks.

Note: no real testing yet.