Add anonymous struct declarations

[lerno]

See http://www.c3-lang.org/types/#anonymous-structs

Alternative to

func void set_coordinates(struct { int i; int j; } coord) { ... }

Would be something like:

struct Vec2 { int x, int y }
// A
func void set_coordinates((Vec2) vec2) { ... }
// B
func void set_coordinates(autocast Vec2 vec2) { ... }
// C
func void set_coordinates((Vec2)(vec2)) { ... }

Or something different.