Support ABI encoder version 1.1

[maoueh]

This is a meta issue to track progress of ABI Encoder/Decoder to support ABI version 1.1.

• ABI Encoder/Decoder (1.1): Supports ABI variants (ABI Encoder/Decoder (1.1): Supports ABI variants #59)
• ABI Encoder/Decoder (1.1): Supports binary extension (ABI Encoder/Decoder (1.1): Supports binary extension #60)

[abourget]

Missing will be encoding/decoding to/from bin <-> Go structs for variants.

Proposed addition:

type MaStruct struct {
	Memo string
	Key  VariantNameUint32
}

type VariantNameUint32 struct {
	EOS_Variant int

	Uint32Val uint32
	Name      Name
}

func (v *VariantNameUint32) SetUint32(bob uint32) {
	v.EOS_Variant = 0
	v.Uint32Val = bob
}

func (v *VariantNameUint32) SetName(bob eos.Name) {
	v.EOS_Variant = 1
	v.Name = bob
}

or something similar. Feedback welcome.

[maoueh]

Started initial approach in the library the supports binary + JSON format, see https://github.com/eoscanada/eos-go/blob/master/types.go#L1087.

Usage example: BlockSigningAuthority