This PR introduces a way to define lambdas in the expression language and allows the definition of user defined functions:

(((foo, bar) => foo + bar)(1, 2) = 3) = true

While there is are no "higher order" functions that accept lambdas, this syntax is useful for defining named functions:

{
  "expression.user_functions": {
     "add_one": "(number) => number + 1"
  }
}

This is particular useful in report generation to avoid excessive repitition.

TODO:

• While users can register expressions, PHPBench itself can't currently and if it did it would contaminate the userspace. Perhaps the expression generator should be able to define it's own functions at "runtime" (?)