Reformat ABI implementation, remove redundant Value struct

[ahangsu]

Summary

Remove redundant Value structs in data/abi and start encoding/decoding from type schemes.

See reference design (and implementation) in algorand/py-algorand-sdk#238.

Scope

• Remove abi_value.go.
• Reformat abi_encode.go and abi_encode_test.go to support (and check) the encoding/decoding from type schemes for golang's primitive value.