A feature-flag to change maps representation to BTreeMap

[akhramov]

As of now, maps are represented with
std::collections::HashMap, which serves as a robust default. In rarer instances, opting for a different implementation, such as BTreeMap, might be desirable, e.g. for deterministic serialization (#496).

This change

• adds btreemaps feature flag which changes maps to be backed by std::collections::BTreeMap.

[stepancheg]

I don't think feature flags should change representation of map.

Because for example, changing adding one dependency to your projection should not make your project much slower because that dependency added protobuf with feature flag.

If deterministic serialization is needed, it could be:

• an option in serializer to sort keys before serialization
• codegen option to change field types to BTreeMap (or to IndexMap)

[akhramov]

Good points, thanks for your input.

I went with this solution, because some types shipped with the crate would still have HashMap fields, i.e. the generated ones:

rust-protobuf/protobuf/src/well_known_types/struct_.rs

Line 38 in 4cffee8

pub fields: ::std::collections::HashMap<::std::string::String, Value>,

I guess it is not a dealbreaker for me anyway. I will try to go with the codegen option in another PR.

[akhramov]

Follow up: #696