Support parametrized "Any"

[judah]

We can use google.protobuf.Any to support basic parametric polymorphism. For example, if we define a file haskell_type_variables.proto with custom options:

extend google.protobuf.MessageOptions {
    repeated string haskell_type_var = 50000;
}

extend google.protobuf.FieldOptions {
    optional string haskell_type_var = 50000;
}

Then we can use those options to annotate a type:

import ".../haskell_type_variables.proto";

message Foo {
    option (haskell_type_var) = "a";
    option (haskell_type_var) = "b";
    int32 x = 1;
    google.protobuf.Any y = 2 [(haskell_type_var)="a"];
    google.protobuf.Any z = 2 [(haskell_type_var)="b"];
}

And generate the following type from that file:

data Foo a b = Foo { _Foo'x :: Int32, _Foo'y :: Maybe a, _Foo'z :: Maybe b}

instance (Message a, Message b) => Message (Foo a b) where ...

encodeMessage/decodeMessage could serialize the submessage as an Any, but in Haskell code represent it as a regular (not serialized) Haskell type. This makes it easier to use proto types directly in Haskell (instead of via wrapper types).

Following the above option-based design will require us to support extensions (#27) so that proto-lens-protoc can understand the new options that we add.