Nullable types

[ricardoboss]

null is already a part of the language and many languages have nullable types.

Explicit support for nullable types also opens up the possibility for less error prone code as variables explicitly need to allow null to be assigned to them.

Nullable type declarations might look like this:

string? a = "hello"
string? b = null

number? n = null

list? l = null

[ricardoboss]

Implement this after #46