Function syntax

[WhiteBlackGoose]

Related: #286

The idea is to use the lambda calculus syntax and logic for our functions (lambdas).

Current thoughts

There's no function names (like we do in classic procedural programming). Instead, we first do application, and then if we want, we can substitute it with a lambda. E. g.:

Entity expr = "f[3] * f[7]";
...
expr.Substitute("f", "\\x. x ^ 10 + 5").Simplify()

Current unresolved problems

Syntax for application (call), which

1. Would be intuitive
2. Would be concise
3. Would be non-breaking

[WhiteBlackGoose]

See #495