Given a message:

```
message Foo {
  oneof timestamp {
    int64 created;
    int64 updated;
  }
}
```

Previously the generated classes by crystal-protoc-gen
would allow to set multiple values of the oneof:

```
foo.created = 123
foo.updated = 345

foo.created # => 123
foo.updated # => 345
```

With this patch the sibling oneof-values
will be cleared when one is set:

```
foo.created = 123
foo.updated = 345

foo.created # => nil
foo.updated # => 345
```

Also when deserializing a message that contains
oneofs there was previously no way of knowing
which oneof value is populated.

With this patch we synthesize a new instance
variable by the name of the oneof to contain
the string-name of the member that is populated:

```
foo.created = 123
foo.updated = 345

foo.created # => nil
foo.updated # => 345
foo.timestamp # => "updated"
foo[foo.timestamp] # => 345
```

Note:
* This also changes the constructor semantics
  when multiple oneof-values are given (now the *first*
  value is kept instead of the last)

* Using reserved keywords as protobuf field-names remains
  a bad idea regardless. For example a field called `delete`
  will break ts-proto / protoc-gen-js and probably others

Previously deserializing a message containing `oneofs`
would not set the oneof instance var. E.g. you would get:

`{ oneof_member1: 1, oneof_member_2: nil, my_oneof: nil }`

Now it works as intended and you get:

`{ oneof_member1: 1, oneof_member_2: nil, my_oneof: "oneof_member1" }`

`myobj.myoneof_value` is more convenient than `myobj[myobj.myoneof]`
esp. when dealing with nested objects.

The return-type of `myobj.myoneof_value` is now restricted
to the types of the possible oneof-values. Previously it would
include the types of *all* message-fields (even those that
are not part of the queried oneof).

Now it should actually work. ;)

Note: Resist the temptation to store ONEOF as Symbol's.
It appears to work at first and saves some String-conversions
but wreaks havoc when JSON::Serializable comes into play.

cf. https://github.com/protocolbuffers/protobuf/blob/master/docs/implementing_proto3_presence.md#updating-a-code-generator