Enum primitives and empty enum variants

[ozgb]

This allows an enum represented in Rust:

enum MyEnum {
    One,
    Two(u32),
    Three(i16),
}

to be serialized/deserialized using:

type MyEnum struct {
	Enum BorshEnum `borsh_enum:"true"`
	One EmptyVariant
	Two uint32
	Three int16
}

[gagliardetto]

LGTM