A74 Add Type Casting

[Ph0enixKM]

Add ability to cast types. Absurd casts should always be warned like:

• Text -> Num
• * <-> Array
• * <-> Null

(* - means anything)

let is_done = true
let is_reserved = true
let is_all = is_done as Num + is_reserved as Num
if is_all == 2 => echo 'Done'

Silence warnings if user is doing absurd cast on purpose:

#[allow_absurd_cast]
pub fun parse(text: Text) {
  ret $parse {text} $? as Num
}