Better serde support for Enumerations

[benj2468]

Currently, if a message has a field that is an enumeration, the field is provided as an i32 and tagged with something like the following:

#[prost(enumeration = "ENUMTYPE" tag = "1")]
field: i32,

This makes deserializing using serde difficult, as the field is type i32, but the standard for serializing proto to JSON is to the use name of the proto.

Has anyone encountered this before, or is there any incentive to make this easier in prost_build?

[cynecx]

cc #1081 (That PR implements proper proto-json support which will likely address this)