protoc-gen-go: simplify generated oneof implementation

[dsnet]

The logic to generate support for oneofs are a significant source of complexity:

• It is complexity to maintain in the generator.
• It is complexity in the generated code.
• It is complexity in the proto package as the generated code depends on the following types, functions, variables, and constants: Buffer, SizeVarint, ErrInternalBadWireType, WireBytes, WireEndGroup, WireFixed, WireStartGroup, and WireVarint.
• It exposes a pseudo-internal method XXX_OneofFuncs.

However, this does not need to be the case:

• The proto runtime, since switching to the table-driven approach, uses XXX_OneofFuncs in marshal and unmarshal only to obtain type information about the wrappers.

Since the type information is the only information needed, we can accomplish this in a cleaner way. Instead of adding the method, we add an unexported, zero-length field with all the wrapper types:

type FooMessage struct {
    // other fields of FooMessage as usual
    xxx_oneofWrappers [0]struct{
        Oneof_F_Bool
        Oneof_F_Int32
        Oneof_F_Int64
        Oneof_F_Fixed32
        Oneof_F_Fixed64
        Oneof_F_Uint32
        Oneof_F_Uint64
        Oneof_F_Float
        Oneof_F_Double
        Oneof_F_String
        Oneof_F_Bytes
        Oneof_F_Sint32
        Oneof_F_Sint64
        Oneof_F_Enum
        Oneof_F_Message
        Oneof_FGroup
        Oneof_F_Largest_Tag
        Oneof_Value
    }
}

This has the following properties:

• It removes the need for the exported XXX method (see protoc-gen-go: unexport XXX_ fields from generated types #276). In fact, this approach keeps all of the type information hidden. Using Go reflection, you can obtain a list of all the oneof wrappers types from the type information in the message alone (no need to dynamically call methods).
• It simplifies the implementation of proto since we don't have to type assert for a method with a relatively complex signature.
• unsafe.Sizeof(FooMessage{}) remains unchanged
• The generated code is significantly simpler.

\cc @neild

[dsnet]

Fixed with https://go-review.googlesource.com/c/protobuf/+/185239