`as` expressions

[thinker227]

Add as expressions for explicit type coercion. This requires adding type names as keywords, probably contextual keywords since they could only be used after the as keyword.

let a = false as string; // "false"
let b = 2 as bool; // true
let c = () as num; // 0

Type names

Type	Name
Number	num
Boolean	bool
Function	func (same as keyword)
String	string
List	[] (same as list expression but without elements)
Object	{} (same as object expression but without properties)
Nil	()

Unanswered questions

• Operator precedence