Implement variable types

[calebhearth]

Assigning to a variable implicitly sets the type of that variable. Subsequent
assignments to that variable cannot be objects which are not of the original
type.

x: 1 # x is of variable type Integer
x: 2 # allowed, since 2 and 1 are both Integers
x: "2" # TypeError

Explicitly typing a variable on declaration sets the variable type. Here, y
cannot be assigned any value that is not a string.

String y: "bananas"
y: 37  # TypeError

Explicit typing overrides assignment, meaning that the following is invalid.

Integer count: "four"

Only the initial assignment to a variable sets the variable type.

Foo:
  # stuff

Bar(Foo):
  # things

Qux(Foo):
  # yet more stuff

baz: Foo() # type of baz is Foo
baz: Bar() # allowed, because Bar is a Foo.
baz: Qux() # also allowed. The variable type of baz does not change after
           # the assignment of a Bar.

void does not affect the type of a variable.

trait: void # trait is untyped
trait: 5 # trait is of type Integer

category: "animals"
category: void # Allowed, but type of category remains string
category: 5 # TypeError

Boolean values are a special case with regard to variable types. Either true or
false can be stored in a variable intially assigned a boolean value, even though
true is typed Dagon::Core::True and false is of type Dagon::Core::False

awesome: true
awesome: false