Use UpperCamelCase for enum values

[nyurik]

I would like to propose protobuf to generate Enum values in a UpperCamelCase to be consistent with Rust's conventions.

Example:

message Relation {
  enum MemberType {
    NODE = 0;
    WAY = 1;
    RELATION = 2;
  }
  ...

would generate

pub mod relation {
    #[derive(Clone,Copy,PartialEq,Eq,Debug,Hash)]
    pub enum MemberType {
        Node = 0,
        Way = 1,
        Relation = 2,
    }
    ...