Support Type Guard on Maybe Types

[Alexius-Huang]

What?
Type guard for maybe types.

Why?
Currently maybe types data cannot be used anywhere except assign it to another maybe type variable.

How?
Sample Wyrd Code

mutable maybe Num foo = 123

Num bar = if foo != Null => foo + 456
          else => 789

Compiled Result

const foo = 123;
const bar = foo !== null ? foo + 456 : 789;

[Alexius-Huang]

Blocked by bug #111