Field declarations

[vihanb]

Add field declarations. Will be in-order compiled to the struct. Examples:

public let a: T = b

public lazy let a: T = b

public var a: T { return b }

public var a: T {
     get { return a + 1 }
     set(b) { self.a = a - 1 }
}

public var a: T {
    didSet { ... }
    willSet { ... }
}

[vihanb]

Added parser in #28