Create CallExpression / High-Order Functions

[sys27]

• Create a new result type for functions (Lambda)
  The Lambda object will represent a definition of a function without a name and will contain the list of parameters and the code of the function. It could be stored in the parameters collection or returned by other expressions.

• Add CallExpression
  All functions (except built-in) will be parsed as CallExpression, it will have a reference to Lambda object (the function to call) and the list of actual parameters to pass to the function.
  (f := (y) => sin(y))(pi) -> CallExpression(Lambda, Parameters)
f(pi) -> CallExpression
sin(pi) -> Sin

• Update parser to support lambda functions.
  (x) => sin(x) / lambda = '(' parameters ')' '=>' expression
f := (x) => sin(x) new type of function declaration???
  (f(x) := sin(x))(pi) vs ((x) => sin(x))(pi)

• Update parser to support inline lambdas

• Remove UserFunction
  UserFunctions will be reworked (partially replaced by CallExpression and Lambda). They will be stored in the existing parameter collection instead of a separate one. For example (the parameters collection):
  pi -> 3.14 (NumberValue)
  f -> (x) := sin(x) (Lambda)

• Support High-Order Functions:
  f(a, b) := a(b)
f(sin, pi)

• Update TypeAnalyzer, Simplifier, Differentiator

• Add references to all existing functions.